I plan to travel round Southeast Asia, and I will obtain a 60-day single entry Thai Tourist Visa in advance. However, on my first entry to Thailand, I only intend to stay a week or so (certainly less than the 30-day visa exemption period). It is on my second entry I would be in need of the full 60 day visa. Is it possible to enter on a visa extemption even if one has a valid visa?
Yes you can.
When presenting your passport, if the officer sees the visa, tell them you're only staying for two weeks this time, and will use your visa next time you enter, meaning you're only applying for a visa-free entry this time.
You will then get the usual entry stamp letting you stay for 30 days.
During the inspection, make sure the visa isn't endorsed with a "used" stamp