Mag length will be your biggest issue mate. I am sure they can can handle 338 lapua with some mods to the mag well but i think the edge with its longer coal will not fit . Looked at it myself some time ago...
Where you planning on running a magazine or single shooting them?
Because the extractor in your Vanguard is already a magnum I would suggest going with the .338-.300 Ultra Mag and single them in. Velocities are very comparable between the 2 cartridges so why not go with the one that fits with the least amount of alterations and $$.
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