Fixed!

Traced HVAC vacuum supply line to 3 way splitter from vacuum pump; isolated HVAC supply circuit and HVAC worked properly. Added remaining vacuum circuits individually until HVAC reverted to Defrost mode. Traced vacuum circuit that resulted in the HVAC defrosting and found cracked rubber vacuum line connector below air filter housing bleeding air into the system. Renewed connecter, vacuum level returned to normal and everything is functional again.

Guys, thanks for your help!