15 slaves/turn is fine for forging and some blood sacrificing but you won't get too far with summoning. I'd say 100 blood slaves/turn for a heavy blood nation is too low. When playing MA Aby (good, but cap only, blood hunters) I'm looking for at least 150, preferably 200. With Mictlan (which I haven't played) I'd expect even higher numbers. When playing Van my goal has typically been 75-100/turn (if I can get GoNB up then Van's horribly cost-inefficient blood hunters become more tolerable and I aim for higher numbers
). Of course map size affects these numbers but this is roughly what I'd aim for in an 8-12 player game.
As for Van post-SDR, I'd suggest using all of your B2 Vanadrotts for blood hunting. Additional, you'll want some blood boosters around and when they are not being used in forging/combat give them to Vanjarls to get a few extra blood hunters. I haven't played any post CBM 1.6 games but I can't see using Vanjarls without a booster/SDR - just too cost inefficent at that point. So, if by turn 50 you've split your cap recruitment 40% dwarves and 60% Vanadrott's you'll have about 7 B2 Vanadrott's hunting and maybe another 3 Vanjarls with boosters. This doesn't seem that bad but the fact that you can't offload most blood hunting duties to Vanjarls means that those B2 Vanadrott's will have to stay focused on that and also be your main options for forging, summoning and combat use (blood magic does have some useful combat spells). I'd suggest summoning Vampire Lords as soon as possible to help with all this.
The thing with Vanheim is that while they are very inefficient blood hunters, and in that sense are a "miniblood" nation, blood magic is very important to them and I recommend making it a high priority and making the best of the current situation (which will hopefully be addressed in a future CBM release).