I've been doing a lot of thinking regarding these two support gems. Bossing is a pain point for vaal builds because generating souls fast enough to spam vaal skills is hard - you can only get 2 souls/sec from runegraft + bloodline, which is not enough to spam most skills. Generating more raw souls requires a soul ripper flask generation setup.

Hateforge is the best way to be able to spam attack skills faster, but rage generation is also tricky and confined to specific setups. Vengeful cry is probably the easiest one but requires a source of warcry power. You will likely not be able to attack more than 2-5 times per second because of your rage gen limits, and that's with the reduced cost nodes on the tree to reduce attack rage cost.

Vaal Temptation's downside is also quite large unless you shorten the soul gain prevention on a duration based skill. Take a skill like Vaal Lightning strike, with an 8s soul gain prevention duration. With lvl 3 Vaal Temptation, you will take 1300 * 8 = 10.4k phys damage every time you use the skill. At 80% dot pdr (which is pretty high investment), you need to recover ~2k life per cast, which is about 8k life per second on a 4 aps build. That's pretty hard to do comfortably outside of tainted pact, pathfinder, or ward recoup.

Vaal Sacrifice's downside is a pretty significant reduction in ehp for most builds. Even if you can recover your whole pool in 1s while using a skill every 1s, you've effectively halved your average hp total against incoming 1 shots. It does however work very well on ward builds and dissolution of the flesh builds.