You need to configure vifm to open `.pdf` files with Evince. In your `~/.vifm/vifmrc` add the following lines:
command evince evince *.{pdf} &
filetype *.pdf evince
The first defines a user command, the second sets the default program for specific filetypes.
The default `vifmrc` (that should be installed to `/usr/share/vifm/vifmrc`) is very well commented and should provide all the documentation you need to get it up and running. The Arch wiki also has a vifm page that contains some helpful pointers and tips.