How can I insert text in the middle of the line to multiple lines in Vim?

Go to the first foo, press Ctrl-v to enter visual block mode and press down until all the lines with foo are marked. Then press Shift-i to insert at the beginning (of the block). When you are finished and press Esc, the inserted characters will be added to each line at the left of the marked block.

To insert at the end, press again Ctrl-v, move up/down to mark all affected lines and then press End or $ to extend the selection until the end of the lines. Now you can press Shift-a to append at the end of all the lines, just like previously with Shift-i.

The visual selection can also be done with normal movement commands. So to comment a whole block in C you could move to the opening brace and type Ctrl-v % Shift-i // Esc.

To answer your first question, the below

:%s/foo/bar(&)/g

will look for foo, and surround the matched pattern with bar(). The /g will do this multiple times in one line.

Since you're just matching foo, you could do a simple :s/foo/bar(foo)/g. The above will work, however, if you decide to match on a regular expression rather than a simple word (e.g. f[a-z][a-z]). The '&' in the above represents what you've matched.