I'm certain there is a reg. which states that one must remain at or above the VASI. coming off an ils at DH you're to transition to the appropriate visual gildeslope. Not sure if it's in the FAR or AIM section.
try faa.gov - thye have all the regs out there.
Common sense dictates that one shouldnt go below the VASI.
class d airspace
(3) Each pilot operating an airplane approaching to land on a runway served by a visual approach slope indicator must maintain an altitude at or above the glide path until a lower altitude is necessary for a safe landing.
for class b it points you to 91.129
(a) Operating rules. No person may operate an aircraft within a Class B airspace area except in compliance with §91.129 and the following rules:
for class c it points you to 91.129
(a) General. Unless otherwise authorized by ATC, each aircraft operation in Class C airspace must be conducted in compliance with this section and §91.129. For the purpose of this section, the primary airport is the airport for which the Class C airspace area is designated. A satellite airport is any other airport within the Class C airspace area.