Are you trying to get a certain consistency or flavor/look?
If consistency, use the vegetable oil, add a little till you get the right consistency
If look/taste, use whatever you want
I don't get the mixing white and dark. Why would you need to?
If you just want to cover pineapple in chocolate, get a double boiler, melt it, then dip the pineapple in
That's how I make chocolate covered strawberries
Or,
Are you putting it into a fondue pot (aka, chocolate fountain)?
If so, use vegetable oil
good luck