It does work when coupled with a dedicated program.
It does not work as you described to get immediate gains for the purpose of having sex with a temporary size increase.
A good program is pump once a day for 10 minutes at vacuum level of -5inHg. If no issues after a week or so move to twice a day. Over time you will see a permanent size increase. You may also be able to get a little post pump size bump for sex. By temp size I mean girth, not length.
Some guys cannot start at -5inHg as it is painful or they get edema or petechia. In that case, drop the vac level to -3 inHg and try again. Drop to 5 min if needed. If no issues after a week or so, increase vacuum level to -4 or -5 and increase duration to 10 min.
Once you have been pumping for a while and have an understanding of how your penis responds you will then have a better sense of how/when to pump with the idea of carrying a little extra size into sex. It may require a little trial and error to see what works for you.
You can over pump and have so much edema that your penis may look large, but in reality is too soft for enjoyable sex.
You also mentioned a cock ring. A cockring can also give you a little bump in girth. It works by constricting the penis and reducing the outflow of blood. Your penis will turn a darker color as it is filled with “old” blood and you need to limit the time u der consteiction to 20- 30 minutes to avoid an injury.
I use one 100% of the time. When I first started using one I was getting about 1/4” bump in girth. My first one was an o ring from a hardware store, figured it was a proof of concept. Tou may need to try a few sizes to determine which one works best for you. The o ring is pretty thin and quickly becomes painful as it cuts into you. Once I determined what size I wanted I ordered a “real” c ring from Amazon. They are thicker than o rings so are much more comfortable. Still, it does begin to get uncomfortable after a bit.
My process is to not put it on until just before intercourse. Obviously this is also one of the areas you need to figure out what works best for you.
Note there are different types of cock rings. I like the firm rubber (silicone) kind as they provide sufficient constrition and allow for ease of removal. The jelly types are very elastic and do not constrict enough for me. Other types like metal or leather concern me as I wasnt sure I could get them off quickly enough.
You may want to do the trial and error while you are still away so when you do return home you can get right into sex with full confidence.