The VTP will look at the highest revision of the information and then all the switches will install that VLAN table. In other words, the latest changes will be used. Caution: this can delete all created VLANs and stop all traffic on your LAN. If done the right way, you will have two switches as VTP server that can add/delete VLANs.
the possibility of deleting all VLAN definitions you mentioned is the same for VTP servers and clients. It's a general rule: Never connect a new switch with higher VTP config revision number to your network!!! There is an easy workaround: change the VTP domain name to something else (bbbbbb, eg.) and back to your domain name. It will make the revision number zero and you can connect the switch to your network without any problem.
To the origional question:
It's recommended to have at least two VTP servers in your network as a backup for the case of the primary VTP server failure. The difference between VTP server and client is the client doesn't save the VLAN definition database to it's config (this is not 100% true, IOS-based Catalysts are using vlan.dat file on their flash for this purpose) and doesn't allow you to change the VLANs. So if you are having only one VTP server in your network and it fails, you are in a little trouble (you still can change one of the clients to a server).
You can even get to a big trouble: let's imagine all your switches are CatOS based, only one VTP server, the network goes down because of complete power failure which crashes the VTP server nvram. After power repair, all the VLAN definitions are lost.
So it's a really good idea to have more VTS servers in your network.
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