The original is the best version. "vital to" is a set phrase and the preposition "to" must be followed by a noun such as "comprehension" or a gerund such as "understanding."
The second version with "vital for" sounds odd. Google N-grams for English fiction shows that "vital to" is four times more common than "vital for."
The third version is grammatically wrong because "to" is a preposition in this context, and so, the gerund "understanding" is required.