I realize the whole polyamory thing encourages that people feel loved not just at a certain time but if possible, most if not all of the time. Then why do I feel so empty that he's away? I feel forgotten.
Because you have a need, when he is away on vacation with the OSO, to have at least a check in. And you don't sound like before he left you laid that expectation out.
You have the responsibility to state your wants, needs, and limits.
Now maybe since this is the first vacaton thingie (is it?) you didn't even KNOW you needed this. So now you do. And when he comes back you sort that out so the next time there's a vacation thingie, you get your check in before bedtime. (or whatever other time.)
Is words of affirmation one of your love languages?
Does he know that then, if it is?
In the meanwhile, he's on vacation. What's sitting around moping do for you? Take yourself out of yourself. Do the hobbies you like, see your friends, have some dates. You don't have to put your own life on "pause" waiting for him to wander back in so you can hit the "play" button again.
Tend to yourself.