Like e.g. on the combat knife?

How big is that bonus, if any? I couldn't find an explanation in the New Vegas wiki.

Pretty sure it just means more than a x1 base multiplier of your critical chance, not an additive bonus.

Thanks

Is it 1.5x, 2x...? Any reference?

It depends on the weapon, there's isn't any set value. If you want to find out, just open up the GECK and have a look.

Or go on the wiki, much easier

It does not seem to be discussed in the NV wiki.

The critical hit chance is listed for each weapon, the ones with a chance of more than x 1 should correspond to the weapons that list a bonus critical hit chance in game.

The critical chance multiplier multiplies the chance you'll score a critical hit over and above what you get from your character's innate ability. For example if you have 5 Luck you have a 5% chance or scoring a critical hit. If the weapon you use has a x1 critical chance multiplier you'll have a 5% chance of doing a critical hit for bonus damage. If the critical chance multiplier for the same weapon is x2 then you'd have a 10% chance of scoring a critical hit for bonus damage.

With the right combination of items, perks, traits and maxed out Luck you can raise your innate critical chance quite high. With some weapons in the game you can actually reach the point were you're guaranteed to score a critical hit with each attack. How important this is depends on the kind of character you want to play. A successful sneak attack will always be upgraded to a critical hit irregardless of your innate ability or your weapons multiplier so if you want to play a stealthy sniper or assassin you don't really have to worry about it.

http://fallout.wikia.com/wiki/Fallout:_New_Vegas_weapons this link list all weapons in the game. sneak attacks will always yield the critical bonus. luck will not be a factor.