Hommer,
It's been awhile since I had a Vanguard, but a 470Capstick round sat in the action with room to spare. Magazine box would require opening no doubt, I don't think the action does ... but could be wrong.

For me ... the Rem700s in big bores suck! Been there and done that, ergonomics are wrong for me as I wear the bolt knob on the trigger finger knuckle on firing. As a 375H&H (had 2) they are simply awesome.
Cheers...
Con