There's a lot that could help you out. CP300 is a sort of frustrating level. You're powerful enough to start hard content, but it can get rough in places. Anyway...

- as a vamp stamblade you might always struggle. You're dealing with being a squishy class assuming a squishy buff in exchange for some mediocre perks. Like you get 10% stam regen. Ok...but the slots you're going to fill with health regen to make up for being a vamp could offer way more stam regen than you get as vampire. Your mist form is weaker than dark cloak. Bat swarm only does magic damage. And if you're addicted to speed, honestly being a non-vamp in night silence hear is far more powerful than a fast vamp...it's another case where the slots used to compensate for vamp exceed the slots needed to get that same perk elsewheee. But the game should be fun, and if you like being a vamp be a vamp...plenty of good players make vamp stamblades work.

- suck it up (ha, vamp pun), you need vigor. You don't have to pvp...pick an empty campaign like Azura's Star on PS4 NA. No one is in there so you won't get notices. Or join as guest...I don't think you get notices on a guest campaign. Do the town dailies for 300 AP, repair walls with kits you buy with gold for 60 AP each...but most of all do the repeatable resource capture quests and practice taking enemy resources. This is really good practice for vet content, and you get 1500 AP for the capture, then another 1,000 if you're carrying the quest. You need 90k AP for vigor...you can get it in 8-10 sessions while never fighting human players. It's just too powerful for a vamp stamblade to pass up.

- I like Hundings Rage as a primary set. when I was a lower level vamp magblade, I ran vampires kiss. It really helped. You give up a lot of power, though. Another set might be briar hearts...I love the set on my stamblade and the heals might help overcome the vampire health regen problems.

- the Troll King monster helm set might be a way around the health issues all together. If you put heals on yourself, especially small HoTs, it'll proc all the time.

- you're at a level where maximizing gear starts to really add up. Running a 5/5/2 config will make you more powerful. Upgrading to gold as you can will make you more powerful...especially weapons, where the dps jump is huge for gold.

- alcasthq.com has really good builds. I think his current stamblade dw/bow "rampage" build would work for you with possible a few adjustments for vamp. This is a little distinct from the conventional "attack and retreat bar." His setup is about damage at range, then sustaining damage up close. You get the dps rolling at range, then when they get in close you switch to dual and stack the HoTs those abilities offer. A bar like that would really help a vamp.

As you continue leveling and improving your gear, a good test to see if you're ready for harder group stuff is running some of the 4 player dungeons yourself on normal. If you can't do Banished Cells 1 and Elden Hollow 1 yourself on normal, your build needs work. If you can do City of Ash 1 or Crypt of Hearts on normal yourself, you're ready.

I'm always happy to answer stamblade questions. But use @davey1107 in your reply if you want me to see it.

- my stamblade needs multiple sets of armor for different content, just like you mention. Yes...it's a pain to change all the time, lol. What I've learned is to use a "base set" (for me, Hundings) and just make all the damn pieces in it...all armor, and all the weapons I use. Then if I collect a specialty set I know I need the jewelry and any two other pieces. I use Hundings and night silence for thieving. If you want to be a super thief, you can use night silence and night mothers embrace from deshaan. Your Khajiit will be pretty dang invisible. For vet content I need stronger gear. I use Hundings and briarhearts a lot.

- I get the pvp hate, lol. It's not for everyone. So some tips to help with you concerns

A. Don't ever pay to leave a campaign. Just don't go into it for three days, or until it flips, then you can leave for free. I'm not sure why they made it so expensive.
B. You can set a campaign as guest. You will not receive notifications for a guest campaign. You can change your guest campaign (if you ever need to) for free under the 3 day rule...just stay out, and wait until the change is free. A guest campaign is the exact same as a home campaign for what you're wanting to play.
C. You can pve in any campaign, but it'll still be a pvp zone so people might try to murder you, lol. There are empty campaigns on any platform...for PS4 NA this is currently Azuras Star. I go in there and see one other player an hour. If you do run into an enemy, the universal gesture for LEAVE ME ALONE I'm QUESTING is to face them and proc your block a few times rapidly, then hold block. Most experienced players understand this and will leave you alone. Sometimes they'll be *** and kill you...whatever, lol.
D. This will allow you to complete important pve in Cyrodiil, notably getting the 49ish sky shards, completing the 12 dolmens, getting lore books, etc. You can get the AP you need for vigor (and caltrops at 180k AP) by taking resources, repairing walls in friendly keeps or doing the 50 dailies in the five towns. This is the slow method.
E. The fast method is to select a busier campaign for your guest (still no notices). Then put out a zone chat "lfg." Get into a 10-20 person group and follow them around. You don't need a mic. You don't need skills. Just help out where you can standing watch, or sniping from walls. Capturing a keep and three resources gives 10,500 AP minimum. I earn 200k AP from a light night of pvp...a group will get you skills and skill points pretty quick. Plus, even if you don't have a mic turn on group voice chat. In large groups people are always sharing tips...you'll learn a TON if you end up in a group with some knowledgeable, helpful, often high, players.

Stamblades are tough, no question. But I always tell people ware struggling with one that if you learn stamblade, it'll make you a ten times better player and you can walk into any other class and pick it up quick.

And new skill bars are hard to get used to. In pvp I've moved to more of a gank bow build, and do a lot of range attacks from walls. So my circle key is changed from retreating maneuvers for speed to siege shield for +2 meters range. The problem is I forget while out in the wild, and accidentally hit siege shield...thereby putting a glowing twenty foot dome over my position. Ugh.

If you need assistance with your bars, I can take a look. I'm not a fan of cookie cutter builds and believe you should use what works for you and what you find fun, but I can take a look and see if there are any glaring omissions.