Don't buy your ticket outside Colosseum. The ticket for Forum is also valid for Colosseum. So, buy your ticket at Forum entrance, where the queue is usually very short. There is one Forum entrance to the west of Colosseum, you'll see it if you walk towards Vittorio Emanuele II Monument from Colosseum Metro exit. When queuing to enter Colosseum, join the line reserved for people with tickets since the same Forum ticket is usable.
As for Vatican, if you're going on a normal day, just go in the afternoon. By then, the tourist crowd would have thinned up. I went there on a Friday afternoon in the middle of summer holidays (peak season hohoho) and the queue was less than 10 minutes, mostly due to security check.
There are a few reasons why you should get a Vatican ticket in advance:
1. You die die must go on "special days", like a Saturday morning, a special-opening Sunday, or the day when Pope is conducting a mass.
2. You want to engage tour guides provided by Vatican
3. You want an off-the-beaten path itinerary, like a tour of Vatican Garden.
Finally, check out this website. He has some good time-saving, crowd-avoiding tips when in Rome.
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