Within the guest operating system, you can change the MAC address to a desired value that doesn't begin with the prefix "00:50:"

What is the guest OS?  If I know this, I can provide some additional assistance.

nmap is putting the VMware label label based on the MAC address.  Each vendor for NICs has specific ranges and VMware is no different.  Here's a list of ranges / vendors that you can refer to - http://standards.ieee.org/regauth/oui/oui.txt.  If you change the MAC in the OS to something out of the VMware ranges - for example to start with 00-05-68, then nmap would check it's mac-prefixes database and come up with another vendor.

Just to illustrate, below is the result of an nmap scan on a test VM, using the VMware-generated MAC address:

########

Re: Nmap revealed some security info about VM?$ sudo nmap -sS -P0 192.168.15.237

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2007-10-21 09:56 MDT

Interesting ports on 192.168.15.237:

Not shown: 1677 closed ports

PORT    STATE SERVICE

135/tcp open  msrpc

139/tcp open  netbios-ssn

445/tcp open  microsoft-ds

MAC Address: 00:50:56:A1:50:34 (VMWare)

########

And, here is the result after I changed the 00:50.. to 00:15.. within the guest OS:

########

Re: Nmap revealed some security info about VM?$ sudo nmap -sS -P0 192.168.15.237

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2007-10-21 09:57 MDT

Interesting ports on 192.168.15.237:

Not shown: 1677 closed ports

PORT    STATE SERVICE

135/tcp open  msrpc

139/tcp open  netbios-ssn

445/tcp open  microsoft-ds

MAC Address: 00:15:56:A1:50:34 (Sagem SA)