There's already a precedent for passive abilities that Liquid Shadow does not interfere with; see devourers. Simply defining the homunculus ability as passive would allow it to work as intended without needing to name a specific card, although as a side effect that would also grant them immunity to lobotomize and allow them to work with other cards that replace active skills such as Butterfly Effect, Mitosis, and Overdrive (!!!). Double growth is probably overpowered.
This needs clarification, though: does the ability get triggered whenever a vampiric creature successfully attacks, or only when directly draining health via Siphon Life? The former is flat-out broken (and I'd assume that was the intent, since you specifically mention making it work with Liquid Shadow), the latter is way too narrow.
On a one-for-one basis vampire homunculi seem balanced. Suppose you play a homunculus + liquid shadow; that's 2 cards and 8 single color quanta for a creature that grows by +3/+1 per turn (you lose 1 to poison damage.) The closest precedent is mono gravity with say armagio + overdrive: 2 cards and 9 quanta for a creature that grows +3/-1 per turn, but starts out vastly beefier. Realistically the armagio is likely to stay ahead of the homunculus until it's too late to even really matter, so that's pretty even.
The problem is when you get multiple homunculi in play. Three armagios + one overdrive still only nets you +3 damage per turn. Three homunculi + one liquid shadow (or hell, just play a vampire) get you +9 damage per turn because they all benefit from the vampire effect. Worse, there's nothing that suggests that the ability only triggers once per turn, and writing it that way would complicate both the card description and the card's implementation in code. But leaving it as is would mean that three homunculi + three liquid shadows gets you an increase of +27 recurring damage a turn. By comparison, if you play 6 steam machines you can get +24 recurring damage a turn (with a temporary one-time 6 extra damage) for the same number of cards and a huge amount of cross-element quanta. Being able to match that in a mono element with no upkeep is pretty broken, and that is by no means the limit of possible homunculus abuse.
