I am using the ViEmu extension for Visual Studio 2015. In Vim I can view the content of registers with :registers or :reg. In ViEmu this returns "Unknown command".
How can the registers be viewed in ViEmu?
The :reg or :registers is a vim extension not found in vi. The ViEmu documentation does not mention this extension. The General section (essentially a summary of changes) makes it plain that ViEmu does not have all of vim's features.
Lacking that, the only way you can "see" a register contents would be to temporarily put it into the current buffer.