^
Indecent proposal eh?

- Do you still have feelings for him?
- You're not even sure if the so called "open relationship" he has with his gf is true.
- Do you really think it will be a 'friendly' vacation, just the two of you especially that you had a past?
- I wouldn't buy his reason that he wants to have a vacation just because he's comfortable with you, I'm sure he has other friends whom he's comfortable with

.
So, my answer would be NO, out of respect to his gf, but then again that's just me. I don't know your circumstances.
