Should I use my 401k to pay off the mortgage?
Great advice by dmurphy. If you are struggling to make the payments then definitely try your bank first to modify your loans. Especially in today's market. I'd even check with your existing lender to see if they can possibly refinance your first and lower that payment to help you. There are...

Can i have a co-signer to help me with my loan
In most cases yet. A co-signer is used to help offset low income and not for bad credit. Remember though, the co-signer's debts will also be considered as well.

I already have one VA loan. I would like to purchase another home,. Do I qualify for a second VA loan?
It may be possible for you to purchase another VA loan with 100% financing. But it will depend on what your remaining eligibility would be. What was the purchase price of your original home? You would also need to move into the new home as a primary residence. So it will have to make sense to the underwriter...