It was a vision of a future reality...
Mat 17:9 Now as they came down from the mountain, Jesus commanded them, saying, "Tell the vision to no one until the Son of Man is risen from the dead."
Not if the scripture is taken in context with itself. In the case of MT 17:9 'vision' is horama, and you choose the second definition rather than the first which reads :"that which is seen, spectacle" Thayers lexicon specifically identifies Mt 17:9 as a type verse for that definition.
This is supported by the parallels in Mark and Luke:
LK 9:36* And when the voice was past, Jesus was found alone. And they kept it close, and told no man in those days any of those things which they had seen (horao). horao - to see with the eyes, which is in the perfect tense, and describes an action which is viewed as having been completed in the past, once and for all, not needing to be repeated. It is also Active, meaning subject are participants doing the action, opposite of a dream/vision in which they are not active.
Mr 9:9* And as they came down from the mountain, he charged them that they should tell no man what things they had seen (eido), till the Son of man were risen from the dead. Edio - 1) to see 1a) to perceive with the eyes 1b) to perceive by any of the senses
These three verses all tell a different story and interpretation. It was not a vision of a future reality, but the very present here and now physical vision of the apostles involved.