You can use a cement walkway, driveway, and/or curb for a light duty anvil.

Also, you can straighten a nail in your small vise that doesn't have an anvil. Just clamp it in the vise, either all the way in, or clamp one end with the rest sticking out then hammer that end until straight.

Or, you could just buy an assortment of different nail sizes, then you won't ever need to straighten bent nails. That would be cheaper than buying a bigger vise with an anvil. I have both.