In a contract, "prazo de vigência" is usually translated as "term".
There are multiple definitions - click on the translation that matches your context.
a specified period of time, eg the term of a contract, lease, loan or office
The term of agreement is 5 years.
an obligation, condition or duty that has to be carried out under a contract or an agreement
Most tenancy agreements contain unfair contract terms, says the UK's Office of Fair Trading, which has tightened guidelines.