What should I consider when applying for a Thai visa with uncertain travel dates and accommodation plans?

I am just filling in the DTV now, I am stuck on date of arrival and intended departure, I will be going in September but not sure of date , I was intending to buy a one way ticket , and an onward flight about 3 weeks in to visit Vietnam for a week , then back to Thailand. Coming home for Christmas probably , and then back to Thailand again. I am concerned that they may just issue a one time entry if it looks like I will only be there 3 weeks?? Do you think is would be better to book a return flight and stay for 2 months? Also re proof of accommodation, I usually only book a hotel for a few days at a time as I like to travel around, will this be a problem? THANKS FOR YOUR HELP. R