You've got plenty of time to see Sossusvlei. I detailed my trip in a report below.

Two Weeks in South Africa and Namibia: Joburg, Cape Town, Sossusvlei and Kruger

With an 11AM arrival you could pick up the rental and get to Sossusvlei/Sesriem but sunset easily. Personally, one day in Sossusvlei was enough but you stretch it to two if you do a hot air balloon ride or helicopter ride one of the mornings.

I only visited Sossusvlei but what you do after would depend on your desires. I'm a photographer, so with extra time I would've headed south but most people head over to Swapkopmund but looking at pictures seemed rather boring to me.

Another option would be to head up and do two days in Etosha then back down to Windhoek.