I have no idea what the ratio of vinyl resin, sand and Portland cement is in Quikrete's "Vinyl Concrete Patcher". I would assume it's a propriortary recipe.
"something that is somewhat smooth AND somewhat light". If you use peat moss, sift it to remove large particles. This will give you a smooth mix. To lighten the mix use 1 1/2 parts sifted peat moss and 1/2 part sand with 1 part portland cement. What are you trying to make? That can make a difference in the recipe you use. Have you looked at the FAQ on this forum?
"I was considering using the Quikrete" Which Quikrete product? They make quite a few.
"Quikrete mortar mix (cement/sand/vinyl)" Their mortar mix doesn't have vinyl resin in it. It's usually a mix of portland cement, sand and lime.
Regarding the bonding agent and concrete vinyl patcher, the manufacturer states "Mix only with water. Do not use with Concrete Acrylic Fortifier, Concrete Bonding Adhesive or other bonding agents."
You are correct that you can remove the leaf in 24 hours but you only have to leave them in the water bath for 3 to 5 days days.
What you are trying to make has a bearing on what recipe you use. Many people only use mortar mix for leaf castings, no added ingredients. You can use almost any recipe for stepping stones BUT heavy traffic area should be made from concrete.
Most of us use portland cement in our recipes and add our own ingredients like peat moss or sifted peat moss or vermiculite or perlite or sand or coir. Then you know what's in the mix and can customize to your project.
If you can give us more details on what you are making maybe we can help with your recipe. You mention 2 different products mortar mix and vinyl patcher so I'm not sure which product your talking about either. Hope this helps a little and questions are welcome.

Thanks for the follow up.

I shouldn't have used the term mortar mix- I was actually just talking about the Quikrete vinyl patcher.

I realized later I wasn't really thinking properly about the perlite - which will of course be very chunky, so when I made a batch of leaf castings yesterday I left it out.

I think most of the time I'll be making leaf castings, but I would like to experiment with some other items down the road. (But like you said, that influences what mix you use, so for now I'll concentrate on the leaf castings.)

For the most part, I just like the idea of something that is strong, smooth, and lightweight. The concrete leaves are so heavy (particularly the larger leaves like rhubarb) that I was hoping to lighten them up a little.

What are the leaf castings being used for? Stepping stones? wall hangings? This too will dictate what mix you use. A stepping stone needs to be thicker than a wall hanging. For a wallhanging, you can apply a thin layer of your mix to the leaf, then embed strips of drywall mesh tape then a final layer of your mix. With leaves like rhubarb that have very large veins make sure you reinforce with drywall tape around the veins.

I was just wanting to see the different ways everyone goes about painting their Concrete leaf birdbaths. I don't want to start off on the wrong foot and have them pealing after 6 months. So far I have read to just use acrylic paints and then to seal with either shellac or just a clear spray paint.... Any other suggestions, or comments will be deeply appreciated.
Judy

Hi Judy
I use the Patio Paint brand paint that I get at Michael's or Joann's and I spray with Thompson's water seal. I leave mine outside all year and they are fine. I refresh the paint in a year or so when they start to look a little dull. I've never had any problems with paint peeling. Of course mine are not used for actual birdbaths, just as decorative garden ornaments.

"I'm hoping for something that is somewhat smooth AND somewhat light..."
I was surprised to read this. Unless I've misunderstood what you've written, you want something that casts smoother and lighter than vinyl patch? In my experience nothing is smoother or lighter than vinyl patch for castings.

Re:"let it dry for a few days","let them sit for a day"
Remember concrete doesn't "dry", it has to cure. Plaster dries, concrete has to go through a chemical reaction. Moving or handling concrete too soon and while it's "green", esp. leaf castings, can lead to cracks, breakage and other mishaps.

I have read so much info on the web, yet I still have a problem with my painted birdbaths. Can I ask the exact steps everyone uses to paint concrete that will be holding water most of the time.
Thanks
Judy

I Need Much needed help!PLEASE I'm trying to make personalized Stepping stones for my walkway up to my porch. I was wanting quikcrete but the store must have gave me the wrong mixture because when I got home I noticed it had alot of Gravels in it . I'm wanting something smooth and something that will hold up. I went back and got mortar mix (I was told this will work) I also put a layer of Chicken wire in the middle to make it stronger but I've had them sitting now for 8 hours and it just seems to look too sandy and gritty looking. Will it look better when it hardens all the way? What is the best mix to use when I want a strong but smooth looking stepping stone? thanks

Mortar mix is generally 3 or 4 parts sand 1 part cement if you make your own. Try 2 parts sand 1 part vermiculite per 1 part cement. That will make it lighter and smooth. If that works maybe try 2 parts sand 1.5 parts vermiculite and 1 part cement. Vermiculite is very small and much softer than