You can be vulnerable by sharing your negative feelings, and you can be vulnerable by sharing the positive ones as well.
Tell that you feel happy, accomplished, in love, serene, successful, at peace, lucky, grateful, loved, accepted, at ease. You will be more exposed than you have ever been, and still be in a state of mind that will help you deal with the exposure and familiarize with it.
Training to be more vulnerable does not have to start from your deepest and darkest emotions. Go from a good place instead. It’s not necessarily easier, surely quite as effective.