Prepend only lines with text in vim

Something like this should work:

:%s/^\(.*[^\n]\)$/* \1/

EDIT Since you asked for a breakdown of the regular expression:

:% All lines

s/ Beginning of substitute command; begin pattern

^ Beginning of line

\( Beginning of group we want to preserve. This will be important later.

.* Any number of characters

[^\n] Some character besides newline

\) End of group

$ End of line

/ End of pattern, beginning of substitution

* \1 Insert *, then the first group that we selected on the left.

/ End of substitution and command

Try

:g/\S/s/^/* /

g/\S/ is a range operator (analogous to % except it selects all lines with a non-blank character).

s/^/* / inserts "* " at the beginning of each selected line.

This avoids the issue with @objectified's answer of putting the prefix on the 1st line of a double blank line sequence.

The following appends " *" at the end of each selected line.

:g/\S/s/$/ */

If the blank line are truly empty, containing only NewLine then this works.

:g/./s/^/*

Explanation

:g/ Global Substitute begin search for pattern

. Match any character except new line :help /.

/^ Match start of line

/* Insert the *