iRULE to extract domain info from HORIZON VDI BROKER XML

Try adding some logging to the iRule to see if the URI, METHOD and Content-Length matches. I tested your iRule and in my lab it triggers the HTTP_REQUEST_DATA event.

when HTTP_REQUEST {
    log local0. "uri: [HTTP::uri]"
    log local0. "method: [HTTP::method]"
    log local0. "content-length: [HTTP::header Content-Length]"
    if { ( [HTTP::uri] contains "/broker/xml") and ( [HTTP::method] eq "POST")and ([HTTP::header Content-Length] == 395)} {
        HTTP::collect [HTTP::header Content-Length]
    }
}

when HTTP_REQUEST_DATA {
    log local0. "HTTP_REQUEST_DATA triggered"
     Parse XML Data
    set xmlusername [findstr [HTTP::payload] "username" 22 ""]
    set xmldomain [findstr [HTTP::payload] "domain" 22 ""]
    log local0. "xmlusername check" $xmlusername

    if {($xmlusername contains "user1")} {
        HTTP::redirect https://vdi1.abc.com
    }
}

Output:

Jun  1 14:28:54 nielsvs-bigip info tmm[13519]: Rule /Common/IRULE_XML : uri: /broker/xml
Jun  1 14:28:54 nielsvs-bigip info tmm[13519]: Rule /Common/IRULE_XML : method: POST
Jun  1 14:28:54 nielsvs-bigip info tmm[13519]: Rule /Common/IRULE_XML : content-length: 395
Jun  1 14:28:54 nielsvs-bigip info tmm[13519]: Rule /Common/IRULE_XML : HTTP_REQUEST_DATA triggered