So my vanilla dunkelweizen is finally carbed up and ready to go, so I figured I'd report back on what I found. I brewed the extract + steeping grains version of the Austin Homebrew Supply anniversary dunkelweizen kit. I went with WY3638 for my yeast. I pitched 1 smackpack with no starter (deliberately tried to underpitch a bit) and fermented at 64F. I bottled one gallon with plain table sugar and the remainder with vanilla sugar (1 vanilla bean, split and scraped, in about 2 cups or so of sugar for at least a month or so). I carbed up to a calculated 3 volumes.
And the side-by-side verdict is... virtually no noticeable difference. The nose was the same between the two - big banana, a root beer/cream soda note that also reminds me of sweet cinnamon or cardamom, a hint of clove and a touch of roasted malt notes. Basically reminds me a lot of banana bread. On the palate, the phenolic notes are a bit lower, but they are still there. Banana ester is still predominant and I also pick up a bit of cherry. The only noticeable difference was that the beer primed with the vanilla sugar seemed to carry the sweet spice notes a bit further into the finish. Frankly, I peferred the non-vanilla version because the finish is a bit cleaner.
So, my take-homes from this experiment are:
1) Vanilla sugar is just too subtle to give much of a result in the finished product. At least, it can't hold its own against a flavorful yeast strain. I doubt it would be notable in a beer with big roasted notes like a stout or a porter either.
2) I *LOVE* WY3638. Killer yeast flavor. I will probably brew a dunkelweizen or weizenbock with this strain every year.