I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
The yellow is probably not due to the fertilizer unless it was a weed and feed and thus contained a weed killer product. The more likely cause is soggy wet soil conditions. Such conditions exclude oxygen from the plant's roots and they begin to die. Yellowing foliage follows. If the damage was not too severe the plant should recover as long as the soil does not continue to stay soggy.
Your vinca may also be suffering from a root rot or another disease called aerial phytophthora. Neither is likely to be cureable in practical terms. I suggest if they plants have continued downhill you consider replacing them with a different species of flowers.
Thanks for the question. Please stop in again soon!
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