I am not sure if my mindset has anything to do with it, but I do not believe my tongues are limited to vocabulary but rather to the discretion of the Holy Spirit. I do not put a barrier to them but believe that He is able to pray that which He desires for my spirit to pray.
This being said, when I started speaking in tongues again (after a long hiatus due to peer pressure as a child) the interpreter and I inquired to the Holy Spirit why the tongues were so fluent and smooth and He said that it was because it was "bubbling up for such a long time." I had it since I was a child, though I thought I was making up the words! haha I didn't realize it was the gift of tongues.
The Lord does seem to give people a certain vocabulary that He builds up, though. Its like any gift of the Spirit some people for whatever reason start out with a fully developed gift and others have to build it up over time. The Lord knows why He does this, but I can say that its funny.
I sought the gift of tongues and the baptism of the Holy Spirit all the while I already possessed it. I sought something that I had, little did I know as a child I had been using this gift all along. One day as I mimicked what tongue speakers sound like in a discussion on the Holy Spirit (to give an example of what it sounds like, not mockingly) in front of two tongue interpreters their jaws dropped. "Ben! You just spoke in tongues!" I had no clue. That led to a three night exploration into the gift and fellowshipping with the Holy Spirit, through tongues and interpretation. Fun.