How can I make a stable olive oil and vanilla ganache?
Disclaimer: I've never tried to make a ganache with olive oil, so take that into consideration as you read the following.

First, I have to assume that you're trying to use olive oil to replace the cream that's normally used in making ganache, perhaps to make a non-dairy ganache. If that's not right, please clarify your question.

Ganache is essentially an emulsion of cocoa butter (usually from chocolate) and cream, plus vanilla and/or other flavorings, that is allowed to cool to the point where the cocoa butter solidifies. It's the solidified cocoa butter that gives ganache it's firm texture. The greater the ratio of cocoa butter to cream, the firmer the ganache will be. If you want to replace the cream, you'll need to make sure that you replace all the components that have an effect on the emulsion. Harold McGee's On Food and Cooking explains the structure of ganache:


  The continuous phase of this mixture, the portion that permeates it,
  is a syrup made from the cream's water and the chocolate's sugar.
  Suspended in the syrup are the milk fat globules from the cream, and
  cocoa butter droplets and solid cocoa particles from the chocolate.


He doesn't say it, but I'd guess that the milk proteins from the cream might act as emulsifiers to help stabilize the emulsion, much as they do with butter.

The melting point of olive oil is obviously different from that of butterfat, but since you heat the cream in order to make ganache anyway, olive oil might still work. Given what McGee says, you'll need to make sure that you add an appropriate amount of water so that there's enough syrup to suspend the butterfat, cocoa butter, and solids. How much is an appropriate amount? Cream is usually 20-40% butterfat, so starting with a 1 part oil for 2 parts water seems reasonable. You might also need to add something that'll act as an emulsifier, like lecithin.