as an enchantment, similar to a 'cleaving' weapon.
A feat
Feats should not be given out as magic items, and there are too many that do it already.
But if you must, look at the ioun stone that gives the alertness feat. That costs 10,000 gp. So at least that much.
rough rule of thumb I have seen is that a feat is generally priced at about 5,000k on a stand alone item, or as a +1 bonus on a weapon. This is based solely on example and extrapolation though, and should probably vary a lot depending on the nature of the feat etc.
In the case of vital strike, since it has very limited utility and is generally considered a sub-par feat I might allow a bit of discount.
Going into feat chains would be more complex, and probably is not a good idea.
Feats, in magic item terms are worth about 5,000gp. Slotless items (Ioun Stones, weapons, rods, etc.) are twice as expensive.
So, adding Vital Strike to a weapon should probably cost about 10,000 gp. I don't think this is underpriced--Vital Strike is a weak feat for weapon users anyway (only natural attacks with massive base damage, like a T-Rex's bite, really benefit from it). I do think the Ioun stone that gives Alertness, however, is drastically overcosted, as even though it is a feat, it is one of the least useful feats in the game, and I've never seen anyone take it for any reason.
Well the ioun stone is slotless and thus double the price Dave Justus, unless you make it cost as part of the overall enhancement structure of the weapon then it is essentially a slotless bonus.
I don't think I would charge more a flat +5000 for vital strike on a weapon (assuming I decided to allow the item to exist in the first place.) Certainly 10,000 or a +1 bonus (and I'd take the 10k if I had a choice as a player first) seems too expensive for what you get. I might even be tempted to go for 4,000 gp.
Obviously, any GM can, and should make their own call, but That is about where I would put the value of this ability. Even then, I suspect few players would find it an attractive investment.
Vital Strike : +1 bonus enchantment. Weapon does 2 times normal damage.
Improved Vital Strike : +3 bonus enchantment. Weapon does 3 times normal damage.
Greater Vital Strike : +5 bonus enchantment. Weapon does 4 times normal damage.
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Why so high.
Because the enchantment would let anyone who uses the weapon do the damage without requiring the Prerequisites that the feat requires.
Which is +6 BAB for Vital strike, +11 BAB for Improved Vital Strike, or +16 BAB for Greater Vital Strike.
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best used on Two-Handed Melee Weapon like: (examples):
Great Axe = normal (M) 1d12 , VS 2d12, IVS 3d12, GVS 4d12.
Scythe (druid yaa) = normal (M) 2d4, VS 4d4, IVS 6d4, GVS 8d4.
Greatsword = normal (M) 2d6, VS 4d6, IVS 6d6, GVS 8d6.
Falchion = normal (M) 2d4, VS 4d4, IVS 6d4, GVS 8d4.
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Anyway, that how i would price it.
If you keep in the requirement that the user still has to meet the BAB requirement of Vital Strike.... then, ya i do the Weapon Bonus enchantment at +1, +2, +3.... But who want to find a weapon, with an enchantment, that they have to wait till the meet the BAB requirement for, (( and for which some classes will never qualify for every )).
anyway my 2 cents worth.
Just make sure it comes with the "usable once a round" clause and "as a standard action". Or just "you get the benefits of Vital Strike". About 5000 would be a good price, considering it isn't a superior feat as pointed above.
Otherwise, as worded above, you are doing probably 1/3-1/2 extra damage on a martial character from just a +1 on a weapon on a full attack. You, do you think you're a wizard? No, then put that down and be sensible, here, fail this will save.