Gotta' get that powdery mildew under control. Once accomplished, things should straighten out, bloom wise. A rose under stress (PM), does nothing right, poor flowering being chief among them. However, when it's super hot, in the 90's+, flowers, unless from a super cast iron blooming variety, hardly last 3 days. That's just the way it is during hot spells.

Cool nights with hot dry days and little overhead rainfall favor powdery mildew conditions to pop up. Not every year will powdery mildew be an issue, but when it comes, it's mostly a spring or fall event, but can come any time conditions favor it, summertime, too.

Please accept some criticism. Your container, very nice looking as it is, is too small for the long haul, IMHO. Consider an up size to a good 10 gal. or better pot if your future plans are to container grow your Vanessa Bell. If it's destined to be planted next year after winter storage, the pot's size is OK. For future container rose growing I suggest you use the best potting mix you can afford, making sure it is heavy in aged/composted pine bark, rather than heavy in peat moss which most potting mixes are. Peat moss breaks down, decomposes, much faster than pine bark. Too much in a potting mix, and PM dries it out more quickly, even shedding water, or it turns the mix into a gummy mess as it decomposes, rotting roots.

When it's hot, watering frequently, more than you thought, is the rule, and mandatory. Over watering will immediately result in the bottom leaves on the canes to turn yellow and fall off. This is the surest sign of over watering. You can hardly over water a rose if the potting mix is a good one and drains well.

The Bayer Advanced Disease Control for Roses.....will work well for the PM. Some rose gardeners like Spectricide Immunox better for PM, but I have no personal experience with it, having never used it.

Never spray when its sunny or too hot, choose evening or early morning to spray. Never spray a rose that is wilting, or looking dry, dehydrated, from lack of enough watering.

Moses

Last year I had good results using a milk water spray for powdery mildew. I plan to do it again when it shows up in my garden.

https://www.thespruce.com/how-to-use-milk-to-control-mildew-1402739