Vanguards bond looks bogus due to being "poorly written".
It looks like it was written early on then studied target changed halfway during development, as if early on the slayer gained a +2 bonus every so many levels similar to a ranger. Now, we are left with an ability you gain at 4th level that doesn't even grant a bonus to your allies until level 10. That said a reasonable GM would let you default to a +1 bonus to your allies as soon as you get it. Hopefully someday Paizo will get back to this and improve the wording to simply grant the full bonus -1 (min 1) rather than 1/2 lvl putting it more in line with existing support abilities like the Bard's Inspire Courage.
To answer the other half of your question I believe the bonus to your allies would apply to their checks, and weapon and damage bonuses. I don't think it bestows a slayer ability on them so the DC probably would not matter. Even if they had their own slayer ability they would likely have to be several levels behind you for your 1/2 level bonus to trump their own.
