Hi Richard
I would go with the advice given above. Stain treatments are a different animal to carpet cleaning and great care is required. Vanish Oxy Powder (or virtually every product with the term OXY, including acne treatments) is an oxydising bleach. Forget what you see in the adverts or read on the labels, the potential for damage to your furnishings is enourmous.
My advice
IF YOU MUST clean the carpets yourself is to use a good upright vacuum cleaner with a new filter bag and clean dry filters and pre vacuum your carpet much more slowly and thoroughly than you would ever dream of doing normally. Even when you've finished, vac key areas again. This should take away about 70% of the soil.
All of your cleaning solutions should have a Woolsafe Approved mark on them. You can then be sure that they will be reasonably effective and safe to use. Measure the amounts to be used accurately. More product doesn't equate to a better clean. Carry out all tests as per manufacturers recommendations. Unless stated otherwise on the label, mix up a stronger solution (about 1:10?) and fill a garden hand spray bottle. Wear rubber gloves and a mask if required. Mist this onto the more heavily soiled areas. Spray some more onto a Microfibre cloth then agitate the area just sprayed gently. If pile damage looks likely, stop agitation. You could use brushes, but this increases the risk of pile damage. Leave to stand for about 5 to 10 minutes. Use your Rug Doctor to rinse away the soil as per manufacturers instructions.
It's worth baring in mind that, collectively, in a typical home, the carpets are the most expensive things after the house and the car. For single items it's usually the 3 piece suite. With that sort of capital investment, is it really worth taking a risk by doing it yourself?
Good luck.
Safe and happy cleaning
Ken