My point is that 'her q is a spell' is highly misleading and utterly irrelevant to the question of whether it does (or should) apply spellvamp (or lifesteal).
Her Q is a spell that empowers a subsequent AA. Most spells that empower a subsequent AA still allow that AA to apply lifesteal. Many also allow lifesteal to be applied to the bonus damage they provide to that AA.
Poppy's Q is a particularly special case that is not intuitive, not clear from its description, and has significant ramifications.
It helps no one to spread misinformation on how it actually functions or why.