Inline deduplication on and compression off is pretty much the answer for every deduplication solution, however, some vendors focus heavily on wanting to show "maximum" dedupe ratio, so they suggest turning off Veeam dedupe. If your criteria for using a dedupe appliance is that you want the largest "dedupe ratio" possible, then of course you want to turn off Veeam dedupe. However, in most cases, the better goal is to focus on storing and transferring the least amount of data, as that will give the best performance, and in some cases, even be willing to give up a little dedupe/storage for performance.
To illustrate why some vendors like to recommend disabling Veeam deduplication consider this example. Let's say you are backing up a disk that has 40 identical 256K blocks, that's 10MB of identical data. Now lets assume that one of those blocks can be compressed and deduped 4:1 for a 75% savings. If you leave Veeam dedupe on Veeam will write only 1 of those 40 blocks, since the others are identical, that's a 40:1 savings in network traffic to the dedupe appliance, but the dedupe appliance will only report 4:1 dedupe. If we disable Veeam dedupe, Veeam will copy all 40 of the blocks to the dedupe appliance, thus copying 10MB instead of 256K, however, the dedupe appliance will deduplicate the 40 blocks, and report a dedupe ratio of 160:1. The amount of data on disk remained the same, but the amount transferred across the wire is 40x more. Once again, I recognize the contrived aspect of this example, but I'm only illustrating the point.
Using Veeam low compression can be a nice performance improvement, saving 10-20% in the total amount of data that must transfer across the wire, while increasing the size on disk typically by less the 5% do to slightly poorer dedupe. This 10-20% savings across the wire can really add up to some improved performance if you have many TB of full backups to run. Dedupe appliances can only ingest a fixed amount of TB's an hour and thus this performance improvement can translate into shorter backups, and faster restores due to less data having to be transferred across the wire during restores as well, but at the penalty of some additional loss of storage dedupe.
As with everything, it's all about striking the right balance for your environment.