"Today's dinner is vegetables."
It is acceptable. However, the sentence structure changes slightly in this case because you've put "today" as the topic. So,
今日は夕飯がやさいです。(Dinner today consists of vegetables.)
would probably be more clear with regard to structural integrity.
今日の夕飯は野菜です。is the more elegant solution as it avoids repeating particles (は、が).
One would usually avoid mashing nouns together, as in your example, and instead insert a 読点、とうてん for clarity:
Hope this helps a little. (=