This question is more for my curiosity than anything - everything seems to work OK.
When you create a vignette using usethis::use_vignette(), you get a template that includes something like this:
---
title: "title"
output: rmarkdown::html_vignette
vignette: >
%\VignetteIndexEntry{title}
%\VignetteEngine{knitr::rmarkdown}
%\VignetteEncoding{UTF-8}
---
If you look at %\VignetteEngine{}, you see a reference to knitr::rmarkdown.
As far as I can tell:
knitr::rmarkdown
#> Error: 'rmarkdown' is not an exported object from 'namespace:knitr'
Created on 2019-05-30 by the reprex package (v0.2.1)
What's going on here? 