Try removing the rest of the guard, see if that stops or reduces the vibes. The broken guard could just be magnifying the vibes, could be a sign of a deeper problem.
Do you know how the guard got broken? Guy sells a bike with 1,300 miles on it with a broken guard, might be a slow speed crash, then 'oh $hit', sell the bike.... any drop could tweak parts of the bars, so second step is to start checking for loose bolts, misaligned bits and other stuff. Search for 'vibration' here, you'll find lots of 'could be this' suggestions. If you're not mechanically inclined, might be worth looking for a decent bike mechanic in your area.
I have a 650, so no specific suggestions on your bike other than the guard thing.