If you don't put a counter on it (a clear, physical representation of that choice), then it has Haste.
Of course, if he hasn't done anything else yet, and if he realized quickly enough, we may determine that he wasn't done putting the counter on there - but this gets “squishy”. (I wonder if I could get that term added to the IPG? MTR? :D )
In general, this will be "
Burning-Tree Vandal, go … no, wait, it gets a counter" - and we'll have to investigate just a little to see if this falls into the realm of MTR 4.8, Reversing Decisions (take-backs, informally). If so, fine, add the counter, move along, no harm done; if not, then that Vandal has Haste (and is wondering what good that does him…)
d:^D