Whats the difference? I know that z is the spread that makes PV = mkt rates at each point on treasury curve but how is that different than the nominal spread?
Nominal spread is based on YTM : i.e. the spread is a bond’s YTM - a risk-free security’s YTM. The ZV is based on the treasury spot curve as you said, so it’s not based on YTM
also, know that option costs = z pread - OAS