In Vegas Pro 16 it’s hidden. Hold Shift + click Options/Preferences to see the Internal tab. In “Show only prefs. containing” search: enable legacy. Then you should see “Enable Legacy (Mainconcept AVC/AAC and Intel HEVC) render plugins”. Click FALSE in Value and replace it with TRUE. Click OK. It’ll be .mp4 though as shown in the screenshot from Francois and as in VP 15 and VMSP 16.