I tried to install NERDTree using vim-plug, so when I started vim and run :PlugInstall, I can use :NERDTree command but after going out it was no available, reason is I didn't download plugin cause I used github, so according to documentation you have to use it on demand and wrap it up your instruction:
call plug#begin('~/.vim/plugged') " Specify folder
Plug 'scrooloose/nerdtree', { 'on': 'NERDTreeToggle' }
call plug#end() " Initializes plugins
So your plugins are available now, and you can use your instructions:
autocmd VimEnter * NERDTree
autocmd VimEnter * wincmd p