For a May wedding/vow renewal reception in a barn, I’d probably wear a sundress (or a sleeveless sheath-type dress) & nice sandals w/ a wedged heel.
If it’s cooler, I’d pair my dress w/ a matching lightweight cardigan.
However, that said, there are other things to consider as well - -
Is the event scheduled for daytime or in the evening - for an evening event, I might go a little dressier.
Do you know the couple well? Are they the more casual type?
What did the invitation look like? Was it more formal or more casual in theme?
Where is the event? Like another poster mentioned, in some parts of the country, a wedding reception in a barn would mean a sundress & cowboy boots.
Just because the reception is in a barn doesn’t automatically make it casual.
I’ve been to a wedding w/ an evening reception in a barn which was very casual - men (even the groom & groomsmen) wore jeans & women wore jeans & slacks - it was in the late fall & ended up being quite unseasonably cool.
However, I’ve also been to a wedding w/ an afternoon reception in a barn which was much more formal - men wore slacks, & the women wore dresses.