I messaged Jesse Hildreth, the ILM animator who was responsible for that sublime little dancing Groot at the end and he told me that Vin Diesel definitely did not perform mo-cap for that shot, and in fact it was James Gunn who performed it for the reference footage!
Jesse was also responsible for most of Star-Lord's helmet shots, some close-ups on Rocket's hands and a couple of Milano shots - all cool stuff!