Note though that the fact that vegetables are uncountable is often due to the fact that 'groente' is also the collective term for all vegetables. If you are referring to 2 specific sorts of vegetables it is possible to use the plural, for instance: 'sla en bloemkool zijn groentes', meaning 'lettuce and cauliflower are vegetables'. ('groentes' and 'groenten' are both correct plural forms by the way) This is not the same with 'fruit'. The plural 'fruits' or 'fruiten' does not exist.
This page might be useful to anyone confused about the use of 'groente'/'groenten'/'groentes'. It is in Dutch so it might be difficult for some, but I'm sure it will be helpful :).