If you want a "wow, that's vanilla" character, you'll need to use real vanilla. Oak will give subtle notes of vanilla, but won't approach the "vanilla porter" level of vanilla intensity you'd get from beans. Charred American oak is the primary flavor in bourbon. But like in bourbon, there are a lot of other spicy/woody flavors happening, not just vanilla.
Oak in small amounts (1/2oz per 5 gallons) can increase structure and flavor stability without tasting obnoxiously like oak. I add some amount of oak to almost every beer I brew. Heavy toast American is especially nice with an IPA. I got the idea after drinking Great Divide's Rumble.
As far as what you're trying to do, for 5 gallons I'd recommend starting at 1oz for a few weeks and going from there. 2oz is quite a lot. As you increase the oak amount, the flavors don't seem to scale linearly. I find 1oz to be pretty balanced. As I head toward 2oz, toasty/woody flavors seem to be increasing faster than the vanilla flavors. YMMV and so on.
I entered a BDS I over-oaked in a comp once. I used 2oz of med+ French oak chips, and about a 4 gallon batch, after racking losses. It was actually one of the better beers I've made, but so completely changed the character it was unrecognizable as a BDS to the judges. A few of the judges said it was more like a Chilean pinot noir.