I just got back (well, last month) from a 13-day trek in Virachey NP to the Yak Yeuk Grasslands (aka Mera Mountain). This is a very tough trek, but for those who want to get up into one of the most remote corners of Cambodia, you can't beat this. Furthermore, there is a waterfall inside the park which we could not get to the bottom of. The guides had never seen it before, but we learned from others back in another village that it is called Sa-a-Na Falls. My trekking partner, a professional ornithologist named Howie Nielsen, tallied up 163 species of birds, and we saw and/or heard gibbons, pig-tailed macaques, gaur, a cobra, Oriental Pied hornbills, Great hornbills, Wreathed hornbills, and several other large birds. Howie just published his trip report (I also have a notation at the bottom of it) on the environmental Web site Mongabay. You can read the report and see pics from that trip here.
I also recorded a short video showing and explaining the panorama from the top of Mera Mountain. You can see that here. Still lots of adventures to be had in Virachey NP up in Ratanakiri.
Actually, I'm counting the first and last days in the village as part of the trek, so it's 11 days in the forest. Rice-wine drinking, bathing in the river -I count that stuff as a part of the trek. We started off in Kohng Ngok (sometimes spelled as Koung Nouk) Village, which is on the O Lai Lai River. If you're not planning on doing a trek into the park, the forest beyond this village is still pretty good. There is a surprising amount of quality buffer zone forest outside of the park boundary in Voen Sai district, and we trekked for 2 days before we reached the national park (there is a stone marker on an old trail to let you know you're in the park). Gibbons and hornbills almost every morning. The Voen Sai section of Virachey is a lot less disturbed than the Ta Vang section. I must say that I feel that the Veal Thom Grasslands are more visually stunning than Yak Yeuk, but if you want super-remote and almost completely undisturbed (we saw just one group of poachers -4 guys- in 11 days and heard just one gun shot and no chainsaws beyond the park boundary). Same cannot be said of the area around Veal Thom, although I still strongly recommend that trek for those thinking about visiting the park.