I haven't done any parsing, but I have played my SK a lot with the vampric embrace line and a proc weapon simultaneously. From my experience, vampric embrace seems to proc more often than my weapon most of the time. This leads me to believe it is either on a fixed rate, or it uses dex and has a proc modifier to make it proc more often.
Spell procs are pretty rare on this timeline, so there may not be a wiki page on them. I couldnt find one at least with a quick check. That is why people are suggesting you do your own parsing if you want to know for sure.
Googling "p99 spell proc" came up with this:
https://www.project1999.com/forums/s...d.php?t=284998 , which suggests it is a fixed rate.