You either need to use a wrapper script to replace -f or use a Lua handler in vicmd if your Vifm is the latest (v0.12.1). The latter is preferable and allows handling all modes in which Vifm uses an editor.
There is a sample plugin I use for testing this functionality here. You can copy and modify gvim.lua as nvim-qt.lua to make it work with nvim-qt (also add ['nvim-qt'] = vifm.plugin.require('nvim-qt'), to init.lua).
Replace gvim wtih nvim-qt, change -f to something else (I can't find documentation on command-line options of nvim-qt, in gVim it means "Foreground: Don't fork when starting GUI"). If there is no suitable option, just leave gvim there and you'll have a mix of editors.
To use do set vicmd='#editor#run nvim-qt'.
See this Wiki page for an introduction to plugins.