There is no text in either description of (malevolence or vulture droid) kits that describe a distinction between summoned vulture droids having a " only a chance" to assist, vs the normal vulture droid always assisting.
There is also no way to make any distinction between "summoned" vulture droids or called vulture droid on the battle field, either the enemies or your own, short of keeping track yourself.
Sometimes vulture droids never assist even if the opponent has a buzz droid debuff showing. There is a bug with plo kloons cleanse that "removes" buzzdroids, but fails to remove them visually, creating confusion in battlefield decision making as to whether the unit actually has the buzz droid on them.
I fail to accept that this bug is working as intended, when there are so many other bugs that CG fails to fix or address. Saying it is "WAI" seems like a lazy "we wont fix it" response, when there is nothing in the games descriptions to describe such mechanics to the players.
The summoned vulture droids have nearly always assisted since Malevolence was introduced. Only after cg "fixed" the bug of vulture droids not always assisting, did vulture droids have a significant and noticable decrease in assists. The point was to fix it, not make it worse.