My Audio Technica portable UHF receiver has an xlr out, but my VX2100 only takes a mini-jack input.

My question is: do I need to get a mono or stereo xlr to 1/8" cable?

The cable needs to be wired for balanced mono XLR to unbalanced stereo mini. Search the threads here, there was one just a couple of days ago.

This one is five feet long. It is also availabe in a one foot length, but B&H is currently showing out of stock on that one.

http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=160635&is=REG