Hardware

If you want the vendor's interface, use the vendor's image. If you want OpenWRT's image, you get OpenWRT's interface. The interface is part of the image, they're not separable.

The closest you can get is something x-wrt which, while not the same web-interface, is at least a web interface and not typing cryptic commands into a terminal prompt.

I think it's possible to reinstall the vendor's original binary later, at least on some routers, though the procedure on how to do so may vary, and you'll need to download the firmware from the manufacturer.

well I am sure I will loose the vendor's interface once I put in my customised version...

or even if i update the vendor's image with mine then also the results are likely to be that i will loose the default interface

Well, of course. That's the entire point -- replacing the vendor's programs with your own to get capabilities the vendor didn't give you. Did you think the interface wasn't a program?

i am aware its a program but...

I was just hoping that there are any open source nvram images

If you're confusing nvram with ROM: OpenWRT isn't open source? And if you're not: Why would you want someone else's nvram settings?

We may be reaching the point of confusion. Tell me exactly what you mean by 'interface', exactly what you mean by image, and exactly what you mean by nvram. As I see these terms:

image is mine
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the interface i need it to be default so that the other users who are trained on the default interface need not worry about the new interface of openwrt and hence are not shocked once they get in mesh network
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for nvram we can postpone as we need an fpga board or something to dump it and use it

You already know the answer then, and are just reaching. I repeat: You can't just use the vendor's interface with OpenWRT's core. It just doesn't work that way. The factory interface isn't hardwired, it's part and parcel of the files and programs in the image: Once you overwrite it with OpenWRT it's gone. Even if you extracted it from another image somehow they won't be compatible. OpenWRT isn't the factory image and that's that.

You probably don't need to worry about the nvram. OpenWRT can take care of that itself.

Give x-wrt's web interface a try. Its core is OpenWRT but it has a decent web interface. It might not be as bad as you fear. Worst case, you write up a page of instructions for your users.