Solution 1:

Tautology is the inclusion of words that have the same meaning as other words already used. So, and vital make the following sentence tautologous:

  • Your contribution is crucial and vital

since crucial and vital are synonymous.

Redundancy occurs when a word is used that adds no extra information to the words already used. So the very in very crucial is redundant because crucial already means very important.

Solution 2:

I'll disagree with the other answers here and say that it isn't actually redundant (and is certainly not a tautology).

When you use the word very with any adjective, you are communicating that the adjective applies greater than normal to the noun that it modifies. That is, a very fat man conveys a higher grade of fatness than just a plain fat man. Any adjective that is comparable can be elevated with very.

Even words that include such elevation in their definition can be compared: one person can be more terrible than another, one architectual work can be more superb than another, one object can be more huge than another, and, yes, one person can have more unique qualities than another and thus be more unique.

Crucial is among these. It is trivial to come up with an example where you can have varying magnitudes of cruciality.

We are closing in on our deadline and we have many crucial tasks that we need to complete before we ship our product. That said, we have two very crucial tasks that we need to attend to the most.

Admittedly, this may be bad style and it may seem too repetitive to reuse the same adjective, but this is merely a stylistic redundancy issue, rather than a semantic issue.

Solution 3:

I'd say it is both redundant and a tautology. Another interesting word for it is a:

pleonasm: The use of more words than are required to express an idea; redundancy.