Any ideas on how to prevent the loop?

So just out of curiosity,

when HTTP_REQUEST { 
    if { [string tolower [HTTP::host]] ends_with "host.domainname.com" } { 
        HTTP::respond 301 Location "https://host.domainname.com/uri/page.html" 
    }
}

are you redirecting to this same VIP? If so then you're basically saying

if host == "foo.com" then goto foo.com

What are you trying to achieve? Just guessing here, but are you trying to redirect to a given URL if the user doesn't enter a URL?

when HTTP_REQUEST {
    if { [HTTP::uri] eq "/" } {
        HTTP::respond 301 Location "https:/host.domainname.com/uri/page.html"
    }
}