The two holes are probably rack mounts (two silver bolts in the frame right below the arrow for the noodle):
(image from http://sheldonbrown.com/canti-direct.html)
If they're the posts to which the v-brake is actually mounted, chances are a rim brake won't be able to be lined up with a 700c rim anyway (however, if you get a 700c disc wheel, you may able to line up the rotor with the disc). This isn't worth the expense. You'll also probably lose some comfort as well. It is also unlikely a 700c wheel will even fit in the frame anyway, so the swap probably isn't even possible.
Rim brakes are generally lighter than discs, but you require rim brake compatible rims (some rims are disc only), which may be an additional expense. If you do want to cut some weight for this and you have the mounting posts, you can do this with some cheap v brakes (e.g. Avid Single Digit 5's) and your existing levers if your brakes are mechanical discs or new levers if your brakes are hydraulic. You may also like the disc brakes better if your rims are prone to messes (such as road grime in rain, snow, mud, etc.). You will also need new cables as well probably. This is probably more effort than its worth, so I'd stick with discs.
Presumably you have a 3x7 (3 chainrings on the front, and a 7 speed set in the back (probably freewheel in the back)). Swapping in a double or single in the front wont save you much, and you probably cant fit a bigger front chainring anyway like a road double due to chain+derailleur issues. You might be able to get a smaller cog freewheel /cassette (there is an important difference) for the back, but this isn't necessarily useful (see next point).
Replacing with a rigid fork can cut out weight, but even keeping the fork locked out should be fine for commuting on the road. If you want to go faster, chances are you should increase your cadence and use gears more efficiently.
Slick tires are the way to go, but you already have those. Keeping your bike in good condition on slicks is probably the best thing to do along with a rigid fork swap if you have the cash. You can combine this with a v-brake swap to the front at little additional cost (the v-brake and cabling) if you have mechanical discs and the front rim is rim brake compatible.
If you want something faster/lighter, you may want to look at purchasing a hybrid or road bike instead, such as the Trek 7.2 FX. But gains for this sort of stuff should probably come from increasing your fitness and technique. A 11kg hardtail (i.e. no rear suspension) frame is heavy, but should be fine for commuting if you're reasonably in shape.