When setting rtp on startup, set VIMPERATOR_RUNTIME to match if it doesn't already exist.

vimperator/locale/en-US/options.txt

@@ -604,6 +604,9 @@ plugin/ +
 Example: [c]:set runtimepath=\~/myvimperator,\~/.vimperator[c] +
 This will search for plugins in both "\~/myvimperator/plugin" and
 "\~/.vimperator/plugin"
+
+On startup, if the environment variable _$VIMPERATOR_RUNTIME_ does not
+exist, Vimperator will set it to match this value.
 ____