You shouldn't need to sand the veneer at all. If you've wet it and raised the grain, then all you need is a 'drag sand', literally just pulling some sandpaper across at one edge so it catches and pulls off the loose fibres. P320 or P400 would work for that.
Pressing down with P320 or P400 on veneer would be likely to close up the grain so stain take-up would be patchy.
You wouldn't normally choose to use P180 on veneer, but in Dozy's case it worked. You do need a very light touch though.