That's correct but IIRC Vanguard was the first with instrumentation that was capable of studying the sun and if memory serves the Van Allen belts were discovered by Vanguard..
Nope. Explorer 1 is officially credited with discovering the Van Allen belts. The only instrument it had was a Geiger counter and it kept getting saturated because they had the upper range set too low. Vanguard 1 was just a 3.25 lb beeper, Vanguard 2 was a 24 lb satellite that had an optical cloud measurement device, and Vanguard 3, weighing in at 50 lbs, did send back radiation data that defined the lower edge of the Van Allen belts, but the first three Explorers and three of the Pioneer lunar attempts really sent back the most data defining the belts.