I know I can manually install the ePO agent onto a client, but is it possible to manually install VSE without having to do the push from the ePO server?
Can I create a VSE package with an up-to-date dat file?
Afterward, if I have no connectivity to the ePO server/agent handler, how do I/can I manually update the dat file on the client (just until I get the correct connectivity back to an agent handler)?
You need to create the package using the Installation Designer Product.
This can then be deployed by whatever deployment tool you use to deliver software.
You can also use this to configure the package with custom policies etc.. to ensure correct function straight from install & until you reconnect with the ePO server.
As to the Update does the device have connectivity to the internet ? If so just point it to the McAfee sites as a fallback repository.