Double type advantage vs CP

Having the double advantage is generally superior, but raw CP can easily be better. It's the attack stat that's key, rather than the CP, then it's just a case of mathematics.

A super-effective attack does 1.4x damage and a double-super-effective attack does 1.96x. STAB, if applicable, (it is in most cases) adds a further 1.2x. A Pokemon with exceptionally high attack could then be superior using just SE attacks, or even neutral damage.

In the specific example, the Golem would be the better choice. If the Vaporeon was a little stronger, around 2800cp, then it would edge out the Golem. That's just in terms of DPS anyway. The Vaporeon would probably survive for much longer.

Battle sim can answer some of your questions.

It just depends on what you are fighting. Against the Ho oh, golem would be slightly better than Vaporeon. If you have a 3000 cp Vaporeon it would probably do better than the golem. I may not be the most accurate, but I hope that I helped anyway.

Your question implies a bit of misconception. CP by itself is not a good indicator of your pokemon performance.
- 1500 CP Golem may range between level 18-22 (aprox.)
- 2500 CP Vaporeon may range between level 28-34 (aprox.)
That's quite a level difference. You need to know your pokemon level and attack IV to find out how much damage it's going to deal to the boss. And their moves, of course. Once you know all that information you may calculate which one hits harder.

In this case, because your Golem is quite low level it's probably going to deal less damage per second (DPS) and will also faint earlier than your vaporeon. Or it may turn out that Golem still has more DPS than vaporeon, but because of Vaporeon's higher level and durability it will contribute to deal more damage overall.

As general advice, forget CP when you judge pokemon performance. It is a misunderstanding.