The max hit difference between Mithril Scimitar and Vestas' Longsword is only 43 LP, at 20 Strength. Also the Vestas Longswords' price will rise, due to they're going to be more rare. You would have to be staking 5M~ each time your using it. Not going to happen in the low lvls, just use the mithril scimitar if you've 20 Attack.
As for the stats, I'd suggest you to get 10 Attack, 5 Strength when your only 60 Defence, believe me, you'd still own. When your reach 75 Defence, go ahead and train 20 Attack.
Oh, and one more point to the case of Vestas' Longsword. The point of Defence pures is not hitting high, it's to avoid getting hit. You don't need Vestas', because you'll win anyway. Sure you can have it in your inventory to feel safe, but you won't need it. Again, I speak from personal experience.