Hate to say it but your gear might be a bit limiting here, pushing a viper 180 off the wind with a 5.8 is probably not the easiest even for a relatively experienced sailor. You will really need to look at mast track position either go further forward or back and boom height. Generally mast track forward or boom down brings more pressure on front foot helps you bear away a little easier.
Gestie is right in that you need to keep the sail upright, as you bear away it can help if you bend your knees and sort of crouch but then slowly stand more upright as you feel more comfortable. Use your number 7 stance- pushing front keeping it almost straight and back leg bent but swing your weight (hips) a bit forward. Also make sure your harness lines are long enough at least 26".
Waterstarting- step 1 learn to steer your board with mast foot pressure, walk around in shallow water steering upwind and downwind. Step 2 go into about knee deep water, sit/lie down in the water. Now with your bum on the bottom you should be able to steer the board upwind and downwind using mast foot pressure. Put back foot up on board GENTLY (slamming your heel on board is quickest way to kill them) pull tail of board slightly towards you and let the sail pull you up- so on straight arms have sail as upright and open to the wind as possible. As soon as you get up on board get your weight forward, keeping weight on back foot will swing it into wind. You want to initiate the water start a little off the wind, the more powered up you are the closer to the wind you initiate the waterstart.
Hope this helps