Hello sreejith1986,
"The VMs in our environment are encrypted at the OS level . Does this oppose Dedupe & compression to work in VSAN?"
Do you mean something other than vSAN encryption or VMware VM encryption? These work with Dedupe but I wouldn't be sure aside from these, though in theory I would imagine they do as a block is a block is a block and if it has the same data then it *should* be able to dedupe it.
"The benefits of dedupe and compression are nil and can also be seen as such in the utilisation charts."
Is all/some of your data Thick-provisioned (either at the VM hard-disk level or via Storage Policy OSR=100)? I ask as Thick data won't benefit from deduplication.
What is the current % used space on your vsanDatastore? Deduplication and compression really only tend to come into their own with relatively higher utilisation as the more data on each Disk-Group the more likely common data will be present and can be deduplicated.
Does your data have a high degree of 'uniqueness' or is the data for each VM/disk likely to have a lot in common? Obviously the more unique data is the less it can be deduplicated.
"if not possible, Is it worth then to disable compression & de dupe considering the overhead de dupe & compression has?"
As with near every design decision in IT, everything is a trade-off - if none of the above points explain why you are achieving a net-loss (and/or whether it is feasible to resolve these or not) then do consider disabling this if you are not benefitting from it as it does have other caveats (e.g. one capacity-tier drive failed in a Disk-Group fails the whole Disk-Group).
Bob