you can stick the Prometrium up your hooha. There's no harm in it. Taking it orally will work fine too though.
The common belief is it's stronger when inserted vaginally as it goes directly to the source but it won't increase your blood progesterone levels as much, where orally it will increase your blood progesterone levels but put less progesterone to the source. There have been numerous studies that also say it really doesn't matter.