Yes, I went late last year and used their Left Luggage facilities at the domestic terminal. If I remembered correctly, it was about 70 baht or 80 baht per luggage, per day.
How long will you be in Vietnam? If it is for more than a week or so, you may want to consider storing your suitcases at the hotel you have stayed at in BKK. Their Left Luggage services may come free (rarely for storage of more than a week) or for a cheaper fee than charged at the airport.