We are trying to plan a 40th Anniversary for my husband's parents. They are planning on renewing their vows with a reception following. Right now I am trying to find a guideline for wording invitations.

The wording on the invitations to your reaffirmation/renewal of vows could read something like:

The honour of your presence
is requested at the reaffirmation
of the wedding vows of
Mr. and Mrs. Martin Anderson
Saturday, the second of December
at four o'clock
St. Paul's Church
Minneapolis, Minnesota
and at a reception following
in the Church basement

[you could also include info that this is in honor of their 40th anniversary, perhaps after their names]