How to get shell TAB-completion to work when I redirect grep ag ack output to vim?

I define ag as bash function, to make usage relatively transparent

ag() { [ $# -gt 0 ] && vim -c silent\ SyntasticToggleMode -c copen -q 
Vim: Warning: Output is not to a terminal

Only way out is to kill stuck vim process from another shell. What's going wrong here and how can this bash function be upgraded (or find some alternative) to make it TAB-completion compatible ? EDIT ag doesn't ship any upstream completion

$ [git:master] complete -p ag
-bash: complete: ag: no completion specification