See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
If you are on H-1, sometimes people ask me, I want to maintain status even though I am getting laid off. Can I be on unpaid leave and does maintain status?
Answer is NO. Unpaid leave is not a device to maintain status. However in this scenario you have gone to gone to India and got the stamping done and you got the paid vacation of 2 weeks , but you want few more weeks of unpaid leave and is this legal?
And the answer is YES. It’s when the employer gives you leave unscheduled, normally unpaid based upon your request not because you don't have a project, then for them not to pay you there is no problem. And for them not to pay you especially when you are outside the USA is absolutely no issue.
The only problem is, make sure for the green card purposes the employer is generating enough money to meet the ability to pay wages issue. This is something you have to talk to your lawyers about it. Just tell them make sure if I am unpaid for few weeks it does not affect my green card, ability to pay wages issue. H-1 is no problem but if W-2 is less than the LCA because you are outside USA there is good explanation.
What documentation can you show? The facts that you are outside the U.S.A; so your boarding passes, any other evidences you have been outside the USA any email from you, something inviting or requesting that unpaid leave of absence for personal reason.