The vial would count as one unit only if it's the same dosage (mg) as the HCPCS reports. Vials are not necessarily single units, unless the weight of the medication in the vial is exactly what the HCPCS indicates for a single unit. For example, Albuterol, compounded, unit dose J7609, reports 1 mg per 1 unit. If your Xoponex vial is 1.25 mg, you'd report 2 units (always report fractions of units as a single unit, when giving the entire dose). Since you're giving the whole vial, you'd not report any 'waste'.
Be careful not to confuse ml (volume) with mg (weight). The unit dose is calculated in mg. but sometimes the manufacturer indicates volume (how much liquid is in the vial). You sometimes have to read the dosage instructions to figure out the weight (mg) of the actual drug in order to post your units correctly.
Hope this helps. Pam