VPS hostname, ns1 and ns2 prefix [closed]

Those are standardised WHMCS fields; just fill in the hostname ("computer name") you want your server to have (doesn't have to have a full domain, really) and anything you want on the other two fields (ns1/ns2 would be fine as dummy values) - I'd be surprised if they use the NS1/NS2 fields at all unless you were ordering a domain name at the same time.

They might not even set the hostname on the server, but may use the hostname field for the reverse dns of your VPS's IP (but also, again, probably not) - it's difficult to say. For example, we use WHMCS, but any process involving hostnames, reverse dns etc would be manual anyway - and that's not all that uncommon.