It is possible to have him spawn back to back, double tap him so to speak
. Not sure if new or older bug. Not 100% positive on workaround since it is a long task to complete.Kill the lords and ladies (except one; Lady M is usually down
).. Wait until spawn again kill lords and ladies then Lady M. Then Vulak will spawn right after you kill him the first time. Evo is capable of testing this further to see what exactly causes this, just might take a couple days for spawns to be set 
Have seen this happen twice, first time was a little confused because not all members were on to check spawn time kills. Re did it today with kills last night and the bug was recreated.