I can only relate my experience.  Bob was on it maybe three or four days before he had a puke session.  I was not using the sulfulcrate.  

Stopped the Piroxicam  for two, maybe three days.  Started him on sufulcrate amd that seems to do the trick.  Now, that said, he did vomit ine rime since then.  Took him off the Piroxicam  for one day, kept doing the sulfulcrate anuway, and he was fine again.    So in Bob's case, a respite inbetween  every now and then seemed to do the trick.it 

Hopefully  you'll find the sulfulcrate  does the trick.  Paws crossed!

My directions were "twice a day" and rarely was that practical for me.  I interpreted  that to mean closer to twelve hrs, so check with your Vet.  If he says every eifht, then that's based on Simon's  needs.

My  reality, quite different.

Bob is scared of thunderstorms and won't  eat if there is ANY hint of thunder anywhere!  We've  had them non stop off and in  for weeks!  So that would interfere  with me trying to give him his Piroxicam  at night sometimes because  I didn't  want to do it without food.

For example, he might get a dose of Sul. what I thought would be  about one ot two  hrs ahead of a meal for his "morning" dose, and then an hour or two before his late supper dose with the Piroxicam.  He eats much better at night when it's  cooled down some.  Of course, there would be a thunderstorm  right when it was time for his meal.  So MANY times it might be four or five  hours after a dose before he ate and got his Piroxicam, but he still did fine,  

Clear as mud, right?

Not sure I'm  making sense, but my main focus was getting the sul. in him a couple of hours before the Piroxicam  dose, even if it meant just once a day.  That's when it seemed most important  to coat his stomach prepping for the Piroxicam. 

And the usual disclaimer...not a Vet and not giving  Vet advice. Talk with your Vet about specifics for Simon.