Let's assume we have a Magus 2/Wizard 10 who wants to cast Vampiric Touch spell on an opponent. The character has prepared the spell using one of the Wizard spell slots. The description of Spell Combat and Spellstrike only requires that the spell is on the Magus spell list – which it is. (Failing that, we can assume the spell is being cast using Broad Study magus arcana, though that would elevate Magus level requirement to 6).
Can a Magus use Spellstrike and Spell Combat to deliver Vampiric Touch prepared with Wizard's slot?
First, an official source: Magus, Spell Combat: What spells can I cast when using spell combat?
It's pretty clear: you can't.
About broad study:
This is less clear, but it seems to say that you can use your vampiric touch with wizard slots.