Upvote:2
US visas have an issue date, and an expiry date. The only time-based criteria for the visa is that you can only use it to enter the US up until the expiry date. There are no "valid for X months after first use" style conditions on a US visa.
Presuming your visa is a multi-entry visa (and almost all B2 visas are), then you can use it to enter the US as many times as you want, up to (and including) the day it expires.
Thus yes, you can (and should) use your B2 visa to enter the US for the trip in December. Doing so will not affect the validity of your B2 visa in any way. Attempting to enter using an ESTA/VWP will likely cause confusion, and has an additional cost, so there is absolutely no reason to do it.
Upvote:4
Your question seems to be based on a misconception that your visa allows a single six-month period of admission to the United States. If your visa is a multiple entry visa, then it is valid for multiple trips to the United States, and normally each of those trips will be limited to six months.
If an immigration officer decides that you may be using the visa improperly to spend more time in the US than outside it, subsequent trips may be limited to shorter periods. However, if your second use of the visa occurs after a transit through the United States, this is very unlikely to happen.
It's likely that when you attempt to transit with your visa that the border officer will refuse entry under the VWP and admit you with the visa. This would be a problem only if your visa is good for a single entry. As Crazydre notes, this is very unlikely, since most citizens of VWP countries get 10-year multiple-entry B-2 visas.
If you don't already have ESTA authorization, and if your visa is a multiple-entry visa, then you should save yourself the $14 and refrain from applying for ESTA.