Lambeth and Vauxhall are (just) in SE London. But it is a fair distance to Woolwich, so I suspect that the sisters probably worked closer to home.
There were hundreds of firms making munitions - or parts of munitions - all over London (and the rest of the UK), so you would only be guessing about their place of work.
However, I recently discovered that SOME records of munitions workers have survived - and are in the following place:
Mr I. C. Todd
Civilian Records Management for Director (D S D C )
Ministry of Defence
Defence Storage and Distribution Centre
Tel 01554 822 352
Fax 01554 822 350
Mr Todd is a very enthusiastic historian, and is always keen to help. You will need to give him as much information as possible - full name, date of birth (if known) etc.
Don't expect miracles. It is like stepping back in time, dealing with this place, and you'll get funny replies such as "What's an email.....?"
If you strike lucky, tell me the secret of your success. I have given them a dozen names so far, without any 'hits.' But it has to be worth a phone call.....