Are you in Arizona, or Maine? Geraniums can tolerate only a fair amount of heat/direct sun.

I'm in northern Virginia and we have temps in the 90's and high humidity in the summer.

What you are describing is quite typical of geranium behavior in some climates and during certain weather patterns. I stopped using them many years ago when I needed a predictable display of color. And I do not like they way that they behave when planted outside to face the elements. Others will rave about them, mind you. ;-)

I believe that they do best with as much sunlight as possible in your climate, though some protection from the afternoon sun would be advisable in warmer areas.

I'll suggest a few cultural things you can try to boost your plants blooming prowess. Avoid over fertilization with a high Nitrogen product and try a 'bloom booster' formulation which will have a higher Phosphorus percentage than the other two primary elements. Fertilize regularly. Also, make sure that you are not waiting too long before you remove the spent blooms. Consistent deadheading is very important. A healthy layer of organic mulch can help cool the soil (thus roots) as well as keep the soil moist longer. Geraniums do like their water.

I'd plan for a second round of colorful flowers before too long.

Any good ideas to replace the geraniums at this late summer date (vincas or others)?

I have alot of white + magenta colored vinca(catharanthus roseus) and they love it hot and can tolerate some degree of dryness.
Mine do receive some shade during the early afternoon.
I also have annual Dianthus, 'Ideal Pearl' a white dianthus with a rose/magenta center...boy, this one does not mind the heat and dry.
(I am in a very hot, dry, sandy/gravel area and these two annuals are fabulous)
I use just compost for these along with an organic fertilizer every 3 weeks or so)
Just a few thoughts for you on this rather hot day in NH...

Thanks for your suggestions. I have a few vinca and may need to invest in a few more. Some have done fine, but others pooped out. They do love the heat though.