Good catch!
That's a LOT of sediment there, along with the trashed petcock screen. Not entirely ruling out your vacuum theory, but it's also possible that with that much sediment, the fuel flow was restricted considerably at all times, offering enough fuel for low-mid range riding, but not enough to keep up with the needed fuel supply at higher throttle openings and causing fuel starvation when you hit 60-65 mph.
Ideally, a gas tank flush/cleaning to remove all sediment and then sealing the inside of the gas tank with a product such as POR-15 would be recommended. (Although I've been able to clean the petcock screen as well as the petcock bowl screen (if your's has one ?) add an in-line fuel filter in the fuel line between gas tank and carbs and keep on truckin' Really depends on how much gunk is in there)
Your post doesn't mention whether or not you've replaced the disintegrating petcock screen. If so, then great! If not, then you might want to in order to prevent more bits from clogging the fuel line again. The petcock screen and small O-ring are available at online parts sources such as Motorcycle Parts, Motorcycle Accessories by BikeBandit.com
In any event, I would strongly recommend adding an in-line fuel filter. They're inexpensive, easy to install and worth every penny..........You can buy clear, see-thru ones so's you can see if any sediment is continuing to build up as well as catch that sediment before it gets into your carbs.
Ride Safe............