Can I be happy in my life if I don't follow the vocation Jesus invites me to embrace?

If someone does not follow the vocation for which Christ made them, they can attain a certain degree of happiness in this world and still attain salvation.  However, they will not be as happy or blessed as they would otherwise have been had they followed their proper vocation.  This is why it is so important that everyone discern their particular vocation prayerfully and responsibly.  Of course, there are trials and challenges in every vocation.  To become a priest does not prevent you from experiencing frustration, suffering and sadness in your life.  But there is great joy in laying down one's life for Christ.  Your vocation is Christ's gift to you.  How you respond is your gift to Him.