Quasits with rogue levels. Quasits are so annoyingly pissy if they start having 'fun' with tormenting your PCs, they'll be loathed.
At roughly the level of your PCs I also tossed a Vrock w/ fighter levels and a warptouch sword, two shadow fiends, and a metric ton of manes at my PCs. It would have gone better but one of them used a Holy Word (when one two PCs were actually good aligned).
And you could always have Vhok hire some Yugoloth mercenaries to send after your PCs.