You can purchase an English-speaking tour of the Colosseum right on-site. They run the tours every so often and it only costs like an additional 7 euros or so. Otherwise, the Colosseum is pretty self-explanatory if you have a decent guidebook with you to refer to the important parts of it. I would buy tickets for the Colosseum at Palatine Hill (which is right next door by the Roman Forum), otherwise you will be standing in a pretty long line at the Colosseum.

As far as the Vatican goes, you can get into St. Peter's Basilica without a tour and without much of a wait, but it will be markedly more difficult to get into the Vatican Museum/Sistine Chapel without showing up really early and standing in line outside of the museum. Even so, you will likely be in a one-hour line in the morning. They don't pre-sell tickets or take reservations, so your only choices are to wait in line or to book one those expensive tours that get you past the line without waiting. Once you get in, it'll take a good hour just to snake your way to the Sistine Chapel through the crowds. We had the good fortune of finding a local guide (American living in Rome) at St. Peter's Basilica that gave a free tour of the basilica and then charged 20 euros for a tour of the Vatican Museum/Sistine Chapel if you were interested in continuing (she even had headphones for narration). There are several people like her just standing around in the basilica's square trying to drum up tour patrons. I would recommend hiring one of these guides (the information you get from them is invaluable in understanding the significance of the many works within the basilica and museum.

Good luck!