Fruit is uncountable but vegetables is countable, so we should use less or fewer before them together?
I'll assume you're using fruit in its more common, uncountable sense.
We say "less fruit", because fruit is uncountable in this sentence, and "fewer vegetables" because vegetables is countable. When fruit and vegetables are combined, the rule of proximity tells us that the word nearest to less / fewer determines which form we should use.
First, I see the distinction between mass and count "words" as one that is not so rigidly followed these days, if ever, and thus in certain cases to be regarded as pedantic and prescriptive - English is the language as spoken by native speakers today, not the rules as set down by grammarians a couple of centuries ago. While the broad rules still seems to hold in most cases, in some it seems to be a historical artefact that is less and less an indicator of how people speak modern English - that is fewer and fewer people make the distinction as universally as they should according to this rule. So the important thing is not to think about it as a rule, but think of it in terms of the pragmatics and semantics which can and do differ, particularly in the case of the example you give.
Second, both fruit and vegetable can be used as mass nouns so you can say
less fruit and vegetable (you are talking about broad classes of food not specific exemplars).
Third, both fruits and vegetables can be used as plural count nouns so you can say
fewer fruits and vegetables (but these means eat less variety not less in total quantity).
Clearly the last is not what people talking about the food pyramid and diet might mean - e.g. on a high protein diet I am told to eat less fruit and vegetable, particularly in the evening, because of the high sugar/carbohydrate content. This has got nothing to do with how many types of fruit and vegetable I eat, which is what the last one says.
I would regard the following as being semantically correct but to my ear just a trifle grammatically awkward (as not grouped as a mass noun complex by me)
less fruit and vegetables
On the other hand the following does not have the same awkwardness because veges has more of a mass noun feel to it...
less fruit and veges
cf. eat your veges (which can still be said even if the only vege on your plate is carrots)
Also in some dialects we even lose the plural marker:
less fruit and veg
The following is even more awkward because there is a phonological and experiential aspect to this (both in terms of "less fruit" going together and "fruit and vegetables" going together in this specific order.
less vegetables and fruit
Fewer is not appropriate in the diet case because for fruits it means types of fruit (not pieces of fruit) while for vegetables it is somewhat ambiguous: "meat and two vegetables" means two types of vegetable, but "how many veges?" is answered by "two carrots and a potato", while "how much vege?" is answered by "two scoops of mash".
So in short choose less or fewer according to whether you are talking about quantity or count, and then choose your nouns and their inflections to suit.
Finally, to recapitulate the first point, any word can in general be pressed into service in a different part of speech, or in this cases a different noun category, where there isn't a competing word already in use. Thus I can shoulder someone aside or table a motion, or I can complain there is only so much table for me to use, or there is so little shoulder to cry on, rather than so few tables to use or so few shoulders to cry on (there may only be the one). When this fits the context it doesn't raise the slightest hackle in a native speaker, but the non-native should be cautious because there may be an appropriate word that should rather be used. Thus you can't say he "footed" the ball aside as there is a specialist word "kicked", and you aren't likely to sound natural saying there wasn't enough "foot" in that shot, if you mean there wasn't enough "kick" in that shot (noting that here we have verb -> count noun -> mass noun by metonymy for "energy in the kick")