I have used machetes extensively for decades down here in the south Florida swamps and I strongly recommend for light vegetation and vines you don't go any shorter than 16 to 18 Inches. I also recommend you start out with a couple of different size Tramontinas which are ridiculously cheap.
For example take a look at this page from Macheteman.com:
http://www.machetespecialists.com/trmabr.html
I would recommend you order the 14 inch Bush and the 18 inch Bush Tram and you will be out a whole $14 plus shipping since they are currently on sale. The 14" is close enough in size to the 12" you seem to prefer that you won't notice the difference and those extra 2" make a lot of difference in useability. The 18" you will appreciate more with time especially if you are doing a lot of cutting. The Tram machetes are very thin and light and that thinness is what makes them swing so easily and won't tire you out like some of the thicker machetes will. With the very sharp convex edge all it takes is a very light swing and the vines and grasses will just lop off with no effort. A nice sharp edge will cut with just a very light wrist flick and seems effortless when you get the hang of it. If you are swinging hard not only are you putting your extremities in danger but your technique is wrong and/or your blade needs sharpening.
If you get these what you really need to do on these is to put a very sharp, high and wide convex edge on them and you wil be able to cut all day with just minor touch ups at the end of the day on a ceramic rod, sandpaper wrapped around a stick or whatever you like to use. A Nicholson Handi-file will work in a pinch but tends to leave a rougher edge. I have yet to need a file in the field but keep one in the truck tool box as back up. The convex edge is surprisingly strong and I have yet to roll my edges even cutting down light trees up to 3" to 4" diameter occasionally. I would not use them to baton unless it was an emergency but I would not be afraid to if you did it smartly and used the end near the hilt.
I also recommend sanding and shaping the wood handles until you get them just right in how they feel to you. After sanding, oil the handles down with boiled linseed oil, tung oil, or danish oil and after drying you are good to go.
These machetes are so cheap you will not be afraid to use them hard. Also, if it turns out you don't like the longer 18" one for whatever reason just cut the blade down to 16" or 12" or whatever length suits your fancy. All of mine are modified in one way or another, especially the tip profiles. Most either have slight drop points or golok/parang style squared or rounded off points. I don't like upswept points but that is just me. I have made PVC pipe sheaths for all mine and the PVC sheaths will last forever and are pretty much indestructable. If you decide to buy a sheath and don't want to make one get the plastic military one made for the Ontario 18" military machete, remove the wire belt hanger and run a nylon strap through the slot left in the metal hardware and sew or pop rivet the nylon strap to keep it from falling out of the slot. Do not get a canvas or nylon sheath since with a sharp blade you will end up cutting through it in no time.