1.5 ton needs 600 cfm to work properly. Assuming you have 6” flex and 4x10 or so vents that’s roughly 100 cfm per vent. This is very rule of thumbish as those number can vary a bit depending on many factors If you still have at least 6 vents remaining it should be ok but I would recommend leaving it as your still working with the same amount of sq footage. That room might suffer a bit with only one vent. You could also upsize the remaining vent to compensate.