I've done a vanilla coffee porter with 2 beans. It had a LOT of vanilla after just 10 days (if I remember). If you have more time, you could only use one and occasionally taste until it's at the right level. I'd guess you'll want at least 5-7 days with one bean but it could be up to 2 weeks. If I had one complaint about my beer, it's that the vanilla was such a dominant flavor. It was really tasty and not gross or overpowering - but it was the main flavor. I've toned down my opinions about "flavored" beers to where I just like the additions to baaaaarely be noticeable.
I split my beans and put them directly into the beer, no bag.
Rum sounds tasty but I'd do a taste test with small amounts of beer. I'm a fan of having restraint and not turning beers into experiments on a whim.