Vampire Facial. How often do I need to get them going forward to keep up the results?

Thank you for your question. You state you had a Vampire Facial® to address some minor acne scarring and some skin tightness. You are pleased with the outcome, and you want to know how often do you need to do this treatment to maintain results.
In the absence of photos and some clinical history, I can give you some general concepts we employ in our practice to guide patients like yourself. A little background: I’m a Board-certified cosmetic surgeon and Fellowship-trained oculofacial plastic and reconstructive surgeon. I have been in practice in Manhattan and Long Island for over 20 years.
To clarify the definition of the Vampire Facial®, it is an approach to helping skin quality through microneedling at different depths that’s considered collagen induction therapy. Microneedling is basically controlled injury resulting in skin tightening and improvement. The element that enhances that is PRP to maximize the skin’s response to this stimulation.
To answer your question, we have to look first at your underlying concerns. You said your skin is not as tight as it used to be. I suspect if we are dealing with an element of facial aging, there is a possibility you are also dealing with volume loss deeper than the skin’s surface. We have become much more aware of facial aging variables which are not just loss of fat, but loss of bone, muscle, and the soft tissue. Very often, acne scars appear more obvious and prominent with facial aging.
One strategy might be worth employing we’ve had very good experience with is placement of fillers at the deeper levels of the face, between the bone and muscle we call Structural Volumizing. We’ve treated patients with volume loss who were getting treatment for acne scars who already had lasers and various injectables, but limited to the skin level. When filler was placed at the deeper level, the volume restoration helped smooth out the acne scars considerably.
Generally, if you had minor acne scarring, chances are you should be relatively responsive. There are different types of acne scars, but that’s outside the scope of this discussion.
In terms of timing, with microneedling and Vampire Facial® type of procedure, often I have to look at my patients in a way where I observe how their skin is responding and evolving. A lot of my colleagues like to do my microneedling every two weeks, some like to do it every month, but I prefer to look at the response, and make adjustments accordingly.
There is a wound healing process every time there is injury, whether it’s a poke or an incision. It’s a process, and the most important parts relevant for you are the proliferative phase and the remodeling phase. The proliferative phase is in the first month when the body is generating collagen, blood supply, and all kinds of good things to help repair this injury. The remodeling is the rearrangement and settling out of that response. You can consider these variables in the timing of the microneedling and PRP treatment based on your skin thickness, skin type, and the rate you heal in a way that’s most effective. From that perspective, you can go more frequently, or allow for some time to give your skin a chance to heal, remodel, and plateau before doing another treatment, so it’s about customization.
It’s also important to look at other options to maximize your results. You may consider treatments such as Q-switch laser, or fractionated laser to help further maximize the quality of the skin, and the use of topical retinoids and other things to help the skin quality.
Discuss this with your physician and see what would be optimal for you, and what is achievable given the usual time constraints and resources one can allocate for the aesthetic purpose. I hope that was helpful, I wish you the best of luck, and thank you for your question.

People ask this all the time, and that's my answer:  "How long does your haircut last?"  
The procedure involves the body, which is always aging.  The results are in the eye of the beholder.  Sure, 5 years after one treatment the effect is pretty much gone.  But after 1 year, some say they still see it, some don't.  
We tell patients that they can do the procedure as often as they would like, no differently than if they were cutting and styling their hair.  Some come once a year and others every 3 months.  Combined with the proper physician grade topical skin products, you should see at least 3 to 6 months of excellent results.  
Be aware, not all PRP is the same.  Make sure your provider is using highly enriched platelet plasma.