A couple thoughts:

Is the growing medium constantly wet? Vinca are quite sensitive to too much moisture - you need to keep them moist enough so as not to damage them by drying out yet not too moist.

How deep did you plant the seedlings? Should be planted right at the root line (where the roots meet the stem) with stem not underground - they're prone to rotting if the stems are buried.

@mxk3:
The plants are at the same depth they were when they germinated. I simply removed the 'netting' off the jiffy pods, and planted them in the new medium, same depth.

I check each day for moisture. If they still seem a bit moist, I'll wait until the next day to water, when the medium is dry for the 1st half inch of depth. Should I wait longer?

I grow vinca every year and they do present problems but they are well worth the effort. They hate being wet and seedlings will damp off. I use chamomile tea to spray with and do not overwater. Although it's true you shouldn't transplant deeper than the root level we plant deeper and they do fine. The worst enemy of vinca is planting outside too soon. One really cold snap can stunt them for the entire season. They like sun, heat and not too much water. They are deer resistant and to me are a near perfect plant.

old leaves .. or new growth???

old leaves = transplant shock .. no problem if buds look great ....

new leaves..perhaps water or fert issue ...

dont know where you are.. seedlings would be suffering humidity deprivation in my z5 MI hot forced air furnace house ...

i agree. .i would not use media with fert in it.. i would prefer to control that myself ....

and i NEVER had any luck messing with peat pots ....they are either too dry.. wicking water out of the media.. or too sodden.. and you can never figure out.. where you are.. inside the wad of soil ... let me suggest plastic 6 oz solo cups next year.. potting up to 8 oz if needed ....

and next time .... try a professional level SEEDLING medium ... rather than the catchall bigboxstore stuff ....

so many variables.. so little time..

BTW.. light level sounds right ...

ken

What's the temperature of the area you're growing in? I think of Catharanthus roseus as a true heat loving species. They simply do not function well in cool temperatures....pansies, on the other hand, will thrive.

I don't much like ken's suggestion of using a seeding /seedling mix; I find them much too fine textured to grow plants in for very long.

ANYTHING at a professional level is better than bigboxstore stuff.. w/in which i personally group all scotts products ...

she can do so much better..

or are you going to debate that also???

in my z5.. furnace and all.. your problem MIGHT be.. if your heat kicks down at night.. to say.. 65 ... your air temps.. and therefore your soil temps.. might spend 12 hours too cold for this plant... it wont kill them.. obviously .. but talk about slo-mo ...

ken

I simply disagree with your assessment of a seedling mix. But if you like something with such a fine texture....go for it! It's not a debate.

The seedlings are on a heat mat, actually. The soil temp is in the low to mid 70's. Ambient temperature in the room is low to mid-60's, which I know is low...but I had great success in growing peppers in that area, and I know they love heat as well.

I've always used the same potting mix, and have always had wonderful results with it. I also like the peat pots. Less waste when I plant in the spring.

In a tropical climate Madagascar Vincas will grow in any old dirt, self seed etc. no fertilizer necessary in general. The key is bright light and high temperatures, at least 70F and can be much higher. You can water as much as you like at these temps. Good luck. - Ian.