Sometimes fear of vulnerability can come from not having done due diligence on step 1 (finding partners you can trust). In some cases like that your gut may be telling you 'if I allow myself to be vulnerable to this person they will use that to hurt me' and your gut will be right.
But, if you have found partners worthy of trust, and then you don't trust them, then you a) wasted all that time finding trustworthy people for no especially good purpose.. and b) slow your growth and that of your relationship and leave a lot of unrealized potential there on the table.
Unwillingness to be vulnerable severely restricts the level of communication you can have, and your love will most likely suffer (or at the very least it will be less than what it could otherwise be).
Rome wasn't built in a day. Or so I've been told (I wasn't there). Make mindful attempts to move beyond your comfort zone if you feel you have trustworthy loves.