While in vi how can I pull in / insert / paste the contents of another file

Type the following

:r filename_to_paste

This will paste the contents of the file after the line on which the cursor is present.

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If you need to copy/paste smaller range of lines/block of text from one file to other, you can also do the following assuming one file is opened in vim already

• Open 2^nd file using :sp (split) or :vsp(vertical split)
• do a normal yy (yank) commands in the other file
• do p (paste) command back in the 1^st file, as the register (place where yanked text is stored) is common to both the files.
• Press Ctrl+w twice to switch between splited files.