Actually that is quite a common situation.However if you had purchased a 300win mag with a 24" barrel,the hornady heavy magnum load with the 180gr bullet produces 3100fps.The hornady 300wby load produces 3120fps with a 24" barrel, for a gain of 20fps.The federal 300wby loads produce from 3040 to 3190fps with a 24" barrel for a maximum gain of 90fps over the 300winmag load.The weatherby brand 300wby loads produce 3240fps with partitions with 26" barrels or less than 3200fps with a 24" barrel.So in reality you are gaining less than 100fps over the 300winmag with your vanguard over a 300win mag with a 24" barrel.If you were to buy a 300win mag with a 26" barrel(such as a browning a-bolt)the 300win mah load would gain 70fps to 80fps wich would basically duplicate the ballistics of your vanguard in 300wby.
Another alternative would have been to buy a remington in 300ultramag.The federal data for 24" barrels lists a maximum of 3070fps with the factory 180gr 300ultramag load in a 24" barrel.Since the 300 ultramags come with a 26" barrel ,that velocity will climb 70fps to 80fps which will put it higher than your vanguard in 300wby for a price much less than a mark V rifle.