I have just installed xemacs 21.0-b67, and a sumo tarball dated
on Solaris 1.5.1 (sparc) and RH GNU/Linux 5.2 (i686). Compilation,
installation, and start-up all went great.
On both platforms, after loading viper, I am told:
Symbol's function definition is void: vc-backend
whenever I try to save a file via:
ESC : [w|x] RET
Viper requires some functions defined in vc-hooks.el
It seems that vc.el is not installed at your site.
(Probably a dependency on vc-hooks is missing in the viper package.)