The deal gives Vanguard a bigger presence in Oklahoma, where the company already pumps oil and gas, Vanguard chief executive Scott Smith said. “The assets being acquired are attractive bolt-ons to our Mid-Continent, Permian, and Gulf Coast Basin operations,” Mr. Smith said, referring to Vanguard’s existing drilling operations in Texas, New Mexico, Colorado, Wyoming and several other states. Terms of the deal calls for a subsidiary of Vanguard to merge into Eagle Rock for $474 million in stock and assume Eagle Rock’s net debt of $140 million. Following that combination, Eagle Rock will be a subsidiary of Vanguard. Joseph A. Mills, chief executive of Eagle Rock, said the agreement gives Eagle Rock shareholders a 24% premium over Thursday’s closing price of $2.45 a share. “We have been very impressed with the Vanguard team and look forward to working together to successfully combine our two companies,” he said.