Hi Alexis_Jeldrez
Firstly, on sizing and combining the two functions - the FETB is not the only measure to use - you need to look at the number of jobs per day and whether combining it all on one server will cope with the load. There are good reasons to separate them, but it is not always necessary (and the guides are just that - general information that needs to be weighed with your specific requirements). You should also consider the recovery objective and whether the single server can provide the necessary performance.
There are advantages and disadvantages of virtualising the master - and the particular environment shoulld determine the appropriate action.
A disavantage of the master being virtual and your VMware farm fails, how do you recover? If your environment is large enough that you have two sites and can afford SRM or similar for DR, then this disadvantage is diminshed.
The advnatage of a virtual master is that it allows the server to be resized easily and relocated as required (if the environment permits it).
While third party OST storage may be an alternative - I would have thought that the cost of this (I'm thinking Data Domain or HP StoreOnce) would far outweight the cost of a second server and storage. In addition, you lose the ability to perform deduplication (with NetBackup) at the client, so all the data will need to traverse the network at least to the media server and possibly to the OST storage server (depends on the availability of OST plugin and capability).
The ability of NetBackup with MSDP to perform client side deduplication can reduce the load placed on the MSDP storage server (thus making the combined master/media more possible).
Regards
David