I just set up my first vivarium and got an Exo-terra dual top light fixture, I have 2 T8 Life-Glo full spectrum 15w 6700k, light fixture also has 2 outlets for halogens, but I replaced them with 2 LED 4w-35w bright white light at 3000k.
Vivarium is for orchids mostly, light fixture sits about 20" off bottom of tank strata.
It all depends on which orchids you will be growing and where your exo-terra located ( close to the window or in a basement, etc.). My best suggestion would be to purchase an inexpensive light meter ( I got mine from Amazon for $18) and research the light needs of your orchids. The 6000K and 3700K is spectrum and not the light intensity. I thought when I bought my first light fixture that 2 of 6K bulbs will give me 12K lumens but oh boy was I wrong lol I got like 2.5K lumens out of those 2 bulbs. Right now I have high output T5 lamps ( a 4 light 48 inch fixture from Home Depot) which gives me between 11K and 16K lumens ( my exoterra is next to the window). I do have a vanda and few cattleyas hence high light. If you grow phals, you would not need as much.