Brewed a porter and the plan is to add a vanilla bean. Would it be better to soak in Vodka or Rum. Reason I ask is I don't drink vodka but do have plenty of spiced rum.
I don't think the spiced rum would hurt as you're using such a small amount that it won't have much impact on 5 gallons. And being cheap, I tend to use what I have before buying anything additional, so I say go for it!
How long should it soak before adding to the beer?
You can bottle right away if you wish. I don't think you'd gain much from a secondary here.
Also, how much spiced rum are we talking about? 1oz?
A lot of people use vodka because it is relatively tasteless so it doesn't interfere with the flavor of whatever you are soaking in it. However, with vanilla (or cacao nibs), some folks like the additional flavor that rum adds. It really depend on your preference.
As for your porter, if you are brewing a 5 gal batch, I would recommend using at least 3 vanilla beans in your rum or you might lose the flavor. Also, wait for fermentation to finish before you add it. Active fermentation will "scrub" the flavor away. Keep in mind that vanilla is one of those flavors that fades pretty quickly as your beer ages. I brewed a vanilla porter with 5 beans in October and the flavor has already subsided quite a bit.