Not necessarily from different vendors as much as from different build lots, etc.
The theory is ... items build from the same components at the same time and the same place should fail/EOL at about the same time (all things being equal).
In practice, I have not seen that. If you are concerned about it, getting drives from different factories or lots (but the same manufacturer) should be OK.
I have seen 18 out of 20 gone in 2 weeks period. Those were 20 PCs manifactured by IBM. I guess it must have been the famous Deathstar bug.
I am not sure if I can buy disks of different lots here(I live in Bulgaria, where a rumour is spread, that hard disk on sale are from lots that have failed the tests for robustness).
What I can do is to spread the purchase in 4 weeks period - buying each week a disk. I am not in a hurry on this project.
However I wanted to do this in a 2 weeks period - buying each week one Barracuda and one Hitachy. As I intend to use software raid which in theory can even work with volumes on different channel types - like using one external USB disk and one internal ATA disk, I was not expecting problems mixing different vendors disks.
BTW According to O'relly's book on linux hardware raid, the only reason to choose disks from the same vendor is not to jeopardise performance by combining one less performant disk with a more performant one.
As this is a rsync server - performance is not a requirement.