It appears that when you paste event attributes using a keyboard shortcut, it also pastes the actual event as well. This does not happen following the default right-clicking method of pasting. It does not matter what the shortcut keys are. An interesting note: undoing these actions requires two undos, the first being the pasted event, the second the attributes.
Version 15.0 (Build 177)
Are you sure what you recall via keystroke is the Vegas Pro "Paste Event Attribute" feature. I think you cannot use this one as keystroke. Maybe this is the Vegasaur feature.
VEGAS PEA has a bug (at least with generated media): 
it should not display Paste Options dialog when I use PEA.
Yes, it pastes the source event when using PEA via main menu. It works fine when using context menu
I haven't used Vegasaur for any of the previous versions. What I'm using is the built in 'Customize Keyboard' feature. I've used this exact shortcut for both Vegas Pro 13 & 14 and neither had this issue. In Vegas Pro 15 it does paste the event attributes perfectly, but it also pastes the event, as if I hit Ctrl+V afterwards.
Confirmed here and reported [Ticket#2017083017010687].
This is fixed in VEGAS Pro 15 build 216.