You have a 70% chance of doing nothing, and a 30% chance of summoning a creature which adds almost 0 to the encounter HP budget. If you did this turn one, by turn 3 you (and it) are dead, giving you 2+2.5 = 4.5 total actions.

70% chance of 2 actions, 30% chance of 4.5 actions is 2.75 actions. Even if the summoned Vrock got a guaranteed 3 actions (went after you in initiative order, say) we still get 2.9 total actions.

The summoner Vrock is only strong if it gets lucky

*or*it is given a round to prepare.

I guess I should do the same math for a succibi swarm. You get 20 of them, how does that compare to 3 whip actions?

The raw damage of the succubi swarm is 6.5*20=130, over 6 rounds that is 780. That is quite decent for an action, but the accuracy is only +5.

The real threat is the 20 DC 15 Wisdom saves per round. If you have a "mere" +10 to your saves this is a 20% chance to fail for each one of them; a party of 4 all of whom has +10 to wisdom saves get charmed the first round by this swarm of succubi.

OTOH, I guess anyone who isn't immune to charm is already screwed.