It is theoretically possible to put Oracle into backup mode, take the snapshot, and export the disks all via the command line. Here are the things you proabably need to do (I don't claim to be an Oracle or VMware expert, so input/feedback is welcome):
Oracle Server:
1) Create a "pre-freeze" bat file that contains the necessary commands to put the Oracle VM into backup mode. This bat file will be executed prior to the snapshot.
2) Create a "post-thaw" bat file that contains the necessary commands to put the Oracle VM back into regular mode. This bat file will be executed after the snapshot.
3) Enable some way to access the Oracle VM "remotely" via the command line. (i.e. enable the Windows telnet service, or install Cygwin and OpenSSH, etc.)
4) Create a user (locally or with some authentication service like Active Directory) to be used with the telnet/SSH account. If possible, limit the system access permissions of the account. Ideally only the pre-freeze and post-thaw scripts should be able to be executed from the account.
5) Poke a hole in the VM software firewall if necessary.
ESX Server.
Create a shell/perl script that
1) Connects to the Oracle VM using telnet/SSH without prompting for a password. Passwordless authentication can be set up for SSH, and I think that telnet allows you to initially specify a password on the command line (so that you will not be prompted for it later).
2) Executes the "pre-freeze" bat then terminates the telnet/SSH session.
3) Takes a snapshot.
4) Exports the disk files.
5) Removes the snapshot.
6 Connects to the Oracle VM using telnet/SSH again and executes the "post-thaw" bat.
It should be noted that ESX Server steps 3-5 can be done using pre-existing backup scripts (e.g. VISBU) so there is no need to re-invent the wheel here.
I would be willing to test all these steps; however, I'm not sure what one should put in the "pre-freeze" and "post-thaw" bat files. If somebody could post that information I would gladly post the results from my tests and try to help anybody else wanting to accomplish the same thing...
Note: the above steps will work if the ESX server is connected to a network on which the Oracle server is also connected, or, alternatively, if the ESX and Oracle servers are separated via a firewall with the appropriate firewall rules in place for telnet/SSH connections.