It's always bad to be without ranged units, so I already mentioned above that I make them without invisibility.

That's why, when the game reaches the 6+ week stage, I take assassins instead of stalkers and upgrade the witches.

P.S. Vampirism makes sense to apply only to cooldown-based units or alternate-grade witches. The others either don't show much initiative or are taken down too quickly. And if we're focusing on Summoning, we have another worthy candidate - Phoenix (but there are limitations here - we need to be sure that the opponent won't destroy it before its next turn).