defining an alias
The simplest way would be to give the command an alias and map the keybinding to invoke that alias.
command! via :execute ':open http://via.hypothes.is/' + buffer.URL
map -b <C-x><C-V> :via
You can then execute the command either with the keybinding, or on the command line via its alias via.
executing the command immediately
That said, do you actually need the command to print out on the command line and wait for you to hit Enter? A preferable solution would be to just have the command execute immediately when you execute the keybinding. You can do that by either:
adding <CR> to the end of the command you execute
map <C-x><C-v> :via<CR>
(this solution is portable to Vimperator); or
using the -ex option to the map command.
map <C-x><C-v> -ex via
executing command and printing a message
If for some reason you don't want an actual alias, but just want the command to execute while printing "via" in the echo area, you could use a binding like this:
map <C-x><C-V> -js
\ dactyl.open('http://via.hypothes.is/' + buffer.URL);
\ commandline.echo('via',commandline.HL_INFOMSG);
\ setTimeout(function(){ commandline.echo('', commandline.HL_INFOMSG); }, 500);
The 500 at the end is the delay (in milliseconds) before the message disappears. If you want it the message to stay until you enter a new command or change tabs, just remove the last line entirely.