A few months ago I went looking for vampires (or rather bloodfiends) and found werewolves a few times in the process.

As far as I know the only difference between info from 2014 and today is that neither vampires or bloodfiends are linked to the cycle of the moons (e.g. I saw werewolves out when both moons were 1/2 full) and there seems to be some uncertainty about exactly how long in real-life a day in Tamriel is.

Here's a summary of what I know:

Werewolves and bloodfiends which can infect players are only found in Bankorai, The Rift and Reaper's March and only when it's night in the game. Importantly not all werewolves and bloodfiends in those maps can infect you, only special ones. For example there's a vampire world boss in Reaper's March and several bloodfiends are in his camp, but none of them can infect players. (An easy way to tell is if they're around in the daylight then they can't infect you, or just let them attack you and see.)

All the infectious ones I saw were in pairs and off to one side of a road, but I was riding along the roads looking for them so it's possible I just missed ones which were further out.

They share spawn points and are mutually exclusive - if werewolves are spawning one night then only werewolves will spawn and no bloodfiends, and if bloodfiends are spawning there will be no werewolves. Also if they're killed they don't respawn until the next night they're due to appear.

There are some maps of spawn points around. I'm not sure how accurate they are, I was following one from 2015 and I saw bloodfiends and werewolves at some of the points and not at others but I don't know if I just didn't spend enough time looking. But basically I just set up a loop around a large part of Reaper's March which went past some known spawn points and lots of different areas and rode it every time I was online at night.

Oh and there didn't appear to be any connection between the Sanguine's Black Goat pet's eyes glowing red and bloodfiends appearing. I saw them glow once during the day, and once after I'd already found and been infected by the bloodfiends but they were not glowing when they first spawned.

The difficult part is finding them, getting infected is easy: let them attack you and you'll get the disease. I got infected by both (cured the lycanthropy, kept the vampirism) and in both cases it only took a few hits. (I kept my armour on and didn't do anything special to make it happen, just stood there and let them hit me, then ran off.) It might help to have a buff tracker on though so you can see as soon as you're infected.

Once you're infected you need to find an NPC called Vorundil (for vamps) or Thoreki (for werewolves) outside the region's main city who can start the quest to complete the transformation. Annoyingly they only spawn in your home alliance - so even though my character was infected in Reaper's March he had to go to The Rift to do the quest.

The other option is to ask players to bite you. Both vamps and werewolves who have levelled the skill line to 6 can infect one other player per week. Asking in your guild/s is a good start, but you can also visit the werewolf or vampire shrine (where the quest takes place) and ask someone to bite you there. Most players will do it for free so you shouldn't have to pay for it.