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In this example the spiral mine around the stem was within 15mm below the spot where the stem transitions from being erect to being droopy. I have been finding similar sign on several different unrelated species of host plants this year, and in every case I've found so far where I've actually dissected the affected plant, the culprit has been a fly larva. The easiest way I've found to locate the spiral mine / confirm that the stem has been girdled is to use a sharp knife to peel or scrape (preferably peel) away a strip of epidermis, starting at the base of the wilted area and going down maybe 30 mm. If the stem has been girdled, the spiral mine should be visible near the very top of the still-erect part of the stem once the epidermis has been peeled away.

There is a faint discoloration visible without cutting the stem open.

It sure does look like your plant has been girdled. I think it's really neat that you found this, and I'd be very interested to learn what this turns out to be. The flies that I'm becoming familiar with that do this sort of thing at this time of year all pupate inside the stem, with adults mostly emerging straightaway (i.e., during the summer). So, if you wanted to take a stab at rearing this, that would be something to consider.

Melanagromyza verbenivora is active in upper stems and inflorescences during the summer, and can cause wilting, but so far, it's only known from Verbena stricta.