If you gain a familiar or animal companion from variant multiclassing what spells can you share with it? You don't gain spells from the secondary class, so I'm afraid that the answer might be "None". I guess the answer also might be that you can share any spells you gain from other classes which are also on the spell list of your secondary class.
For instance, could a Cleric with the Luck domain could cast True Strike on a familiar gained from a Wizard VMC, or would only actual Wizard spells work? (which seems like kind of a shame since the PC will probably never have any of those besides a single level 0)
Taking the share spells feature from the PRD:
It doesn't seem to specify that the wizard (I assume other familiar-granting classes or abilities have language that makes the character count as a wizard for the purposes of the familiar or whatever) has to cast wizard spells if he wants to share spells - only that the target has to be "You".
That's interesting since it seems like the rules for animal companions are different from those for familiars. Here's the text from the Druid rules.
"Share Spells (Ex): The druid may cast a spell with a target of “You” on her animal companion (as a spell with a range of touch) instead of on herself. A druid may cast spells on her animal companion even if the spells normally do not affect creatures of the companion's type (animal). Spells cast in this way must come from a class that grants an animal companion. This ability does not allow the animal to share abilities that are not spells, even if they function like spells."
I'm not sure whether the secondary class from VMC should count as its own class or as part of whatever the primary class is. The latter would make the VMC Druid a bit more attractive for other casters. You're basically getting extra class features instead of feats, but I'm not sure what class if any those class features belong to.