- You are probably best off just hitting dat <kbd>Tab</kbd> key 5 times in your example:<sub></sub>  
`:%s/\s\(item.\)/\r`    <kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd>    `\1/g`  
<sub>(The tabs display as `^I` in the Command-line.)</sub>

- More elegantly, you can have vim repeat that typing for you. But with its ≥7 keystrokes and a bit of thinking effort, this technique is only economical for a bigger number of repetitions or characters to be repeated:<sub></sub>  
`:%s/\s\(item.\)/\r`    <kbd>Ctrl</kbd>-<kbd>f</kbd> <kbd>`5`</kbd><kbd>a</kbd> `\t` <kbd>Ctrl</kbd>-<kbd>c</kbd>    `\1/g`

- The most literal and technical answer to your question is – at the cost of even much more verbose syntax – using `\=` for substituting by an expression [[`:help sub-replace-expression`](http://vimhelp.appspot.com/change.txt.html#sub-replace-expression)], in which you could then employ the [`repeat()`](http://vimhelp.appspot.com/eval.txt.html#repeat()) function:<sub></sub>    
`:%s/\s\(item.\)/\= "\n" . repeat("\t", 5) . submatch(1)/g`  <sub>  (Readability spaces not required.)</sub>

Unfortunately, vim does not provide syntactic sugar for repetition (`\{5}` or otherwise) in the replacement part of the `:substitute` command;  almost none of the special regex syntax for pattern matching [[`:h pattern-overview`](http://vimhelp.appspot.com/pattern.txt.html#pattern-overview)] is available for the replacement: [`:h sub-replace-special`](http://vimhelp.appspot.com/change.txt.html#sub-replace-special)

<br/>
<sub>(PS: <kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd><kbd>Tab</kbd>)</sub>