You clearly don't understand how quiescence is supposed to work and seem to think it's only a VMware or hypervisor phenemon. It isn't. It's been around for literally years even before virtualization began.
Also, flushing disks is not application consistency backup, its crash consistency. Databases dont hold just disk buffers in memory. They hold databases logs and only this database itself know what to write to where.
You need to read about VSS basic concepts. Quiescence is useless unless there are writers, and this is exactly what database vendors provide. The command to quiesce doesn't just flush files to disk, it informs these applications that provide a writer that a backup is about to happen. The application then handles the logic to write out any outstanding in-memory I/Os to disk. This writer and requestor structure is the whole reason why quiescence does provide application consistency and not crash consistency.
No, I dont. They dont recommend nothing, I dont have time for this etc. You cant use those pre-post scripts, because you cant take server offline. And usually brand dont give you more specific commands to quiescene. And finally I dont need this crap at all - I can just backup whole VM together with memory dump file and together with snapshots, through iSCSI LUN and thats more faster and easy way to do.
So, again, that isn't the fault of the principle of quiescence, that's the fault of whoever this vendor is. If they don't recommend anything then either A.) they don't need their application/system quiesced and crash-consistency is good enough or B.) they're a crap company and don't care about supporting their users in operation. Either one you pick, this doesn't negate the usefulness or operation of quiescence.
Yes, VM embedded servers do only stuff that they declared to do. Do you think I have time to let them make some special version only for me. Today every brand make its own out-the-box stuff. I have many different brands, all embedded VM-s. They are "embedded" just because its easy to use. Without any mails exchange with support. Just read documentation and thats all. Like it - dont like it, take next brand box.
I don't even know what you're talking about here. The brand of physical server makes no difference, it's the operating system and application which determine how data protection must be managed. You don't ask HPE how to take application-consistent backups of Oracle running on a DL380, you ask Oracle how to do that. The physical platform makes no difference.
I recommend you spend some time on Google reading about how quiescence really works and stop making wildly false claims based on incorrect understanding of these principles.