Hello marcelpan,
Why do you not want to (or can't) upgrade the vSAN cluster to 6.5 U2 also?
And also, why do you want/need to update the vCenter to 6.5 U2 (without updating the hosts)?
If you do this there is one issue you will encounter regarding vSAN capacity overview (Cluster > Monitor > vSAN > Capacity) - this won't report used capacity correctly if the vCenter is 6.5 U2 and the hosts are on a lower version:
"vSAN capacity monitor shows lower Used Capacity than actual capacity used
After you upgrade vCenter Server to 6.5 Update 2, the vSAN capacity monitor (Monitor > vSAN > Capacity) does not include capacity used by virtual machines on hosts running 6.5 Update 1 or older. The Used Capacity displayed is lower than the actual capacity used.
This issue is resolved in this release."
VMware vCenter Server 6.7 Update 1 Release Notes
Other than this, I am not aware of any issues so yes you can do this but do note the above.
Bob
