The two biggest outside investors in the $100 billion Vision Fund are complaining about the high prices the fund’s manager, SoftBank Group Corp., has paid for tech companies and the control wielded by SoftBank Chief Executive Masayoshi Son over investment decisions.

The investors, Saudi Arabia’s Public Investment Fund, or PIF, and Abu Dhabi’s Mubadala Investment Co., have contributed almost two-thirds of the Vision Fund’s pledged capital. If they are unhappy with the fund, it could become more difficult for Mr. Son to raise...