Does this mudra on its own transmute the sexual energy and can theirfore take the place of pranayama?
All of the described rites are a ritual that you perform together with your Divine Mother and the Holy Spirit. The sexual energy that you accumulate in your sexual glads comes from the Holy Spirit. Therefore the Vipakarani Mudra utilizes the sexual energy for healing and is a kind of transmutation. But it is utilized to heal, to restore the body. Ultimately, when your body has healed sufficiently, you might want to use this rite to heal your consciousness, in order to awaken it.
Also, when practicing this mudra I begin to feel a tingling at the tips of my toes. Is this something I'm doing wrong or just an effect of that posture?
Such tingling is most likely something neurological and could be related to your nerves. Please make sure that you have something soft under you back, that does not push or squeeze your nerves (in the back). And don't overdo the posture, if you cannot reach the 90° angle, start with what you body is able to do and then increase the angle with care and time.
The tingling might also be a sign that some nerves heal, which have been previously damaged.