I think I am going to hazard a guess what's going on wrong here. If the user navigates away from the webtop, and hits the virtual again, they will be sent to the login page even if they present a valid MRHSession cookie. You can control that behavior using an irule as well:
when HTTP_REQUEST {
if { ( [HTTP::cookie exists MRHSession] ) and ( [ACCESS::session exists -state_allow [HTTP::cookie value MRHSession]] ) and ( [HTTP::uri] equals "/" ) } {
HTTP::redirect "https://[HTTP::host]/vdesk/webtop.eui?webtop=/Common/portal_webtop&webtop_type=webtop_full"
}
}