Q: Should guests buy a gift for a couple who are having a vow renewal?
A: Assuming you have been invited to and are attending the vow renewal ceremony or reception party a congratulatory card is the most that the couple renewing their vows would expect. Having been married for a number of years it is fair to assume they're not in need of items for a new home or help getting started as a married couple. If you'd like to buy them a gift a token gift such as a gift basket would suffice. These are not newly weds and if they were to expect ‘gifts' for what is still a fairly unique event, it would be ungracious of them. Customs may vary in different cultures and countries so do check further if you're unsure.
Do not confuse a vow renewal ceremony or party with an Anniversary. It is usual to bring a mile-stone related gift to a wedding Anniversary party. See Wedding Anniversary for gift ideas. And of course if a couple is renewing their vows on their anniversary then a vow renewal anniversary gift is welcome.
Below is a small selection of vow-renewal cards and gifts you can personalize. Clicking on any image will take you to the store where you can view a wider selection of vow renewal gifts.