I just split two beans lengthwise, sanitize a nylon bag, put the beans in the bag and throw it into my fermenter after fermentation is complete, or sometimes even just into my keg.
I dont bother really 'sanitizing' the beans other than having starsan on my hands when i am cutting and working with them...they are coming out of a sealed tube..have clean hands and a clean work environment and you shouldn't be getting infections.
2 beans is more than enough for most dark beers, even too much depending on how fresh your beans are.
The good? thing about vanilla though is that it mellows out pretty fast. So if you add too much just wait a few weeks and it will mellow out.
Also if you didnt know Costco has amazing deals on Vanilla beans, dont buy them in the spice area of your grocery store its a ripoff.