Point about 30 DNS Zone NS entries to the same two IP Adresses

You need to create two A records pointing ns1.mydomain.com to the ip address of ns1.vpsprovider.com, and ns2.mydomain.com to the ip address of ns2.vpsprovider.com. CNAME entries add extra dns lookups, and should be avoided when creating nameservers (and infact break RFCs).

If you're using these vanity nameservers for your own domain and not just for other people's domains, then you'll also need to set the ns records for your domain to your own nameservers and the ip addresses of the nameservers provided. You'll want to do all this on your vps provider's nameservers rather than 123-reg's because you'll no longer be using 123-reg for anything other than domain registration.

There are no disadvantages in having a nameserver setup like this other than the fact that you then have to use the control panel of the vps provider to change your dns rather than the 123-reg control panel (though that may actually be an advantage).

I don't recall if 123-reg provide .uk nameservers or not. If they do, and your domain is a .uk domain, then there will be a small theoretical loss of speed by switching to a .com nameserver. However this won't be noticable.

You shouldn't really have to notify anyone, beyond your own domain registrar.

If you intend to have nameservers at 2.2.2.1 and 2.2.2.2, just make certain that they only serve the names for which they are authoritative, and nothing else. Pretty quickly, clients of the old nameservers at 2.2.2.[12] will realize that they're not getting responses, and they'll stop sending queries.

If you don't have nameservers at 2.2.2.1 and .2 (and it's probably easier if you don't), then just let those incoming queries drop to the floor. Again, the clients at the sending end will very quickly get the hint that there's no point sending queries to your particular corner of the internet.