I was surprised to have done it too.
I was in Reapers March and thought I would give it a go - since it seemed no one else was coming to help.
TBH - The main concern for any Vampire players is the fire, and the Worm Cultists tend to have a pyromancer or two in their midst.
Spamming Drain or Destructive Reach on them in sequence usually allows me to keep them off balance long enough for my Devouring Swarm/Bats to passively kill the melee would be attackers.
(Dodging and rolling when they get close)
With Invigorating Drain IV - I tend to recharge Ultimate very fast, especially on Dolmens - since I have also got Fighter Guild upgrades to add more ultimate on Daedra / Undead kills.
So by the time the cultists are dead, I usually just need a few more daedra kills to fire off another bat attack.
Fire Atronarchs are a hassle however - when I am struggling to avoid the Dread Ice Atronarch coming at me, I have to keep running around the middle of the dolmen to avoid line of sight with the fire guys.
Destructive Reach - when it hits, is usually enough to knock the fire guys off balance, Drain has a very high accuracy and while being a DOT/Slow damage, it does tend to kill things reliably.
So in the end, I wiped all the Daedra out except the Dread Ice guy - but he eventually fell too. In panic moments, the Bat swarm was usually enough to get me back to full health.
So just thought I would post this, as I was surprised at how easy this Dolmen was in Reapers March solo, when I still have trouble with the ones in Malabal Tor with groupies.
I did respec my character at the start of the week -
I had made the mistake of spending points in bow and sword - when I should have been pure DPS Sorc.
So now I have devoted all points to
Storm calling : all passives + endless wrath (the storm blast that returns magicka)
Daedra Summoning :Restoring Twiglight - the flying minion +Rebate + Power Stone (is that the ultimate discount?)
Dark Magic: Passive that lowers magicka costs for all abilities
Crystal Shards (one that gives 30% chance for instant cast)
Vamprie abilities - All passives (except the stun ones on drain)
Just drain and bat swarm - have not gone for Mist this time - as it is not really a combat useful ability.
Fighter Guild - All passives : Currently at level 8 I think (have 9% extra daedra/undead damage, + extra ultimate on daedra death)
Nord : +30% health regen (its either +20% or +30% - I am level 38 nord: I know - vampires usually go for dunmer for the fire resist, but I like nords)
Destruction Staff : Havent really invested much here- I used to have the vulnerability to elements ability, and some passives, but I exchanged them just for the Destructive Reach ability.
Gear:
Entire Robes of Necromancer : between levels 29-34, purple. +554 Health on enemy death, occurs at most every 3 seconds.
Legendary Staff of Zymel Hriz (Lightning Staff) (Level 33 - Gold) : 691 Damage, enchantment has damage shield that absorbs 230 or so damage for 5 seconds)
In any case - after I did my respec, I focused on passives that increase damage and increasing my magicka and health regen.
I've noticed a good improvement.
As I mentioned, Im still only level 34ish, so have plenty more skill points to get, plus skyshards.