I'm a fan of MtBaker's Vanilla Cupcake. I get it with 2 extra flavor shots to give it a wonderful intensity.
Most Vanilla Cupcakes I've tried (six or seven total, including DIY) are good. They differ in proportion of vanilla to cake flavor, and in the specific vanilla profiles of each. For instance, CopperCreek's Cake is delicious---it's almost all cake with just a little vanilla and not much sweetness. Most Vanilla Cupcakes are much sweeter, including MrBaker's, probably to add the frosting element. What distinguishes MtBaker's Vanilla Cupcake are the options in base blend (I get mine 20/80 PG/VG) and extra flavor, plus the amazingly inexpensive price. 50mls for $12 shipped and made-to-order? That's a no-brainer.
On the other hand, I'm not a fan of most other MtBaker juices I've tried, 19 in all. Many were pedestrian, and a couple were downright awful. The three winners for me have been Graham Cracker, Kahlua, and Vanilla Cupcake. YMMV.
Among the DIY flavorings, Capella's is my preferred Vanilla Cupcake. TFA's is OK, but lighter in flavor, and FlavorWest's is the least tasty (to me), with an artificial side-flavor. Each one has a slightly different vanilla.