You should get the form before you start (this is how it is for med). I know someone who had to do it, i dont have to until next year (becuase bmedsc and then mbbs) but im thinking of getting it all out of the way now hehe.
I know for med they wanted them fairly early, ie before you started anything clinical which is week 2 or 3 to my knowledge. With dentistry, clinical things start in semester two so I assume they would allow more time. Its better to start early anyway incase youve missed a vaccine such as hep b because this requires 3 shots (start, 1 month, 6 months) to get you to the right levels.
EDIT
This is all assuming that dent vaccination requirements are the same as med. I'm guessing they are the same.