What are the best tubes for a VTL TL 2.5 Preamp?

There are major differences in tubes, both in 12AU7 and 12AX7 types. In both cases my favorite is Telefunken, especially in the 12AX7 type. I think they build the best 12AX7 ever made.

Will different tubes and brands make any difference in sound?

That depends entirely on your system and set up. Your system is not posted here at Audiogon, so I don't know your other components or room set up.

Assuming the rest is equal to your VTL 2.5, I would say yes, premium tubes will be an audible improvement in your system.

Is it worth the investment? That is another matter, especially when discussing Telefunken, which are expensive.

I can say that when I replaced my amp's stock China 12AX7s w/NOS RCAs, the amp immediately sounded MUCH better. And I'm not talking subtle differences here. I would do some research before buying, but you definitely want to get those Chinese tubes out of there.

I agree with Albertporter, and recommend Telefunkens. There's a lot of crap and bootlegs out there. I also recommend Andy at Vintage Tube Services as a source!