You could just make a seperate alias called soldier_primary_2 and have it the exact same but with viewmodels on. Then have a button you don't use bound to toggle the other.
So have
bind "5" "soldier_primary_1"
alias "soldier_primary_1" "soldier_primary; bind 5 "soldier_primary_2""
alias "soldier_primary_2" "soldier_primary_2;bind 5 "soldier_primary_1""
As also said above you may as well remove the interp stuff, it's pointless now.