I use a piece of clear tape stuck to the inside of the loop backed with some blutak pressed against it to create a well with a very slightly convex floor then carefully place the vial into the lanyard hole onto the tape and adjust position with a toothpick.
It’s tricky.
Then once I’ve got the vial in place, level, making sure no vial parts (especially tube ends) are protruding past the light body etc I apply some UV glue with a toothpick by slowly dripping in small drops until there is a layer filling the bottom of the well & surrounding the vial.
To help eliminate bubbles you can apply tiny amounts of glue into the vial ends first before placing as there are small pockets on each end which often like to expel tiny air bubbles at just the wrong moment.
Set with UV light making sure no bubbles are hiding, then slowly apply more glue to fill the well some more and harden again.
Fill to a level that covers the vial and is as flush as possible with the body of the light, again tricky due to the curvature.
Then you can remove the tape, give the inside a clean with cottonbud & ISP etc and apply another thin layer to the backside to help protect the vial some.
Hope that gives you some ideas