You can coduct IMT no problem with the foregrip.. like someone else stated, it sticks out less than the magazine anyway.
I carried via a 3 point sling that held the rifle at the low ready on my front, not the back. Getting "unslung" was a non issue. When I had to sling it over the back (Chow hall) it hung with the left side of the rifle against my back, not the bottom, so it was again a non-issue.
Funny thing.. I was never issued a bayonet except for ceremonies. But.. if I had to fix bayonets and stab someone, the vertical foregrip would only help.