I do not think the NIV does the original Greek justice in this case. The English Standard Version (a respected relatively new essentially literal translation) renders the verse this way:
7 Likewise, husbands, live with your wives in an understanding way,
showing honor to the woman as the weaker vessel, since they are heirs
with you of the grace of life, so that your prayers may not be
1 Peter 3:7
In Calvin's commentary on this verse his translation supports this choice in translation as well. A few other resources I checked seem to align with this as well.
That being said, the word weaker is still there. So what does this mean? Commentators seem to agree that in this passage that the use of the word weaker was used to prompt the husband to act in a gentle and gracious manner.
Moreover, he employs a twofold argument, in order to persuade husbands
to treat their wives honourably and kindly. The first is derived from
the weakness of the sex; the other, from the honour with which God
favours them. These things seem indeed to be in a manner
contrary,—that honour ought to be given to wives, because they are
weak, and because they excel; but these things well agree together
where love exists.
Commentary Critical and Explanatory on the Whole Bible:
Both husband and wife are vessels in God’s hand, and of God’s making,
to fulfil His gracious purposes. Both weak, the woman the weaker. The
sense of his own weakness, and that she, like himself, is God’s vessel
and fabric, ought to lead him to act with tender and wise
consideration towards her who is the weaker fabric
Matthew Henry's Concise Commentary:
Christians ought to do their duty to one another, from a willing mind,
and in obedience to the command of God. Wives should be subject to
their husbands, not from dread and amazement, but from desire to do
well, and please God. The husband’s duty to the wife implies giving
due respect unto her, and maintaining her authority, protecting her,
and placing trust in her. They are heirs together of all the blessings
of this life and that which is to come, and should live peaceably one
with another. Prayer sweetens their converse.
The argument that "weaker vessel" implies care and gentleness carries a lot of weight in my opinion. If you have a fragile, beautiful vase are you going to knock it around and treat it roughly? Not if you care about it. Taking a good literal translation into account and viewing this as an illustrative analogy will help a lot in interpreting this verse.