I recently ran into an issue where drush vset was not setting a string variable (in this case, a time period that would be used in strtotime()) correctly:

# Didn't work:<br>$ drush vset custom_past_time '-1 day'<br>Unknown options: --0, --w, --e, --k.&nbsp; See `drush help variable-set`&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [error]<br>for available options. To suppress this error, add the option<br>--strict=0.

Using the --strict=0 option resulted in the variable being set to a value of "1".

After scratching my head a bit, trying different ways of escaping the string value, using single and double quotes, etc., I finally realized I could just use variable_set() with drush's php-eval command (shortcut ev):