As others have suggested it's probably a dry wick.
1. First, check the PG/VG ratio of your juice. That tank with those wicks needs a thinner juice. 70% or more PG is required for them to work well.
2. Second, you'll probably want to empty the tank, clean it out with distilled water, dry it really well, and then insert a new coil and wick. Your current wick MIGHT still be good, but then again it might be burnt and if you try to use it in a fresh tank of juice it could make the whole tank taste burnt.
3. Third, before you fill the tank, put 2-3 drops of juice directly on the wicks at the center of the coil. Then fill your tank, put the top on, turn it upside down, and swirl it around a little. Turn it back upright and just let it sit for around 5 minutes before using it.
4. Finally, 8 watts might be pushing it a little with that particular type of tank. Drop down to 6 watts, and then gradually increase the power until the flavor starts to fall off or you start getting a slight burnt taste. At that point drop the wattage back down by .3 watts or so and it should work fine from there.
There's nothing wrong with the top-wick style of tank, but you've got to know how to use it. Follow the steps 3 and 4 every time you put a new coil in the tank, DO NOT OVERFILL the tank, and it should work fine. You may decide that this type of tank really isn't for you and you want to switch to a bottom coil tank. If you do then I would suggest switching to the Aspire Nautilus. That will work really well on your Vamo in the 12-15 watt range with 50PG/50VG juices. You'll get a LOT more vapor from it, and better flavor as well.
And remember for the future... No matter what type of tank you get, you ALWAYS need to prime the wicks first. Always put a few drops of juice directly on the coil, and always let it sit for a few minutes to completely saturate the wicks before using it. Taking a few puffs without pressing the fire button before using it also helps.