Ask Mr. Smarty Plants

The disease that is causing trouble for the cultivated garden impatiens (Impatiens walleriana) in at least 33 states is downy mildew. There's a good article in ScienceNews, the online magazine explaining the downy mildew problem on impatiens. There are no native vincas to suggest as substitutes, but there are several native annual flowering plants for you to consider. Some will bloom better with more sun.
To find an annual native plant substitute, the first place to go to find a list of potential plants is our Native Plant Database.  Use the Combination Search feature instead of Recommended Species. This will provide a bigger selection with much more choice to narrow down. The volunteers and staff at the Wildflower Center who maintain the database have partners in different regions to help with these recommended species lists based on what is easy to access in local nurseries.
Under Combination Search, select the following categories: Habit – herbaceous, Duration – annual, Light requirement – part shade or shade, Soil moisture – moist, height 0-3 ft.
Some of the impatiens substitution possibilities include:

Bidens cernua (nodding beggartick) yellow blooms August – October.

Capsicum annuum (chile pequin) white flowers followed by small green fruit turning red when ripe.  Hot, edible fruit.

Geranium viscosissimum (sticky purple geranium) purple/pink blooms May to August.

Mimulus bicolor (yellow and white monkeyflower) yellow blooms April to June.

Nemophila maculata (fivespot) white blooms with purple spots from April to July.

Rudbeckia hirta (black-eyed Susan) yellow daisy-like blooms from June to October.

Salvia coccinea (scarlet sage) white, pink or red blooms February to October.

Tinantia anomala (false dayflower) white, blue or purple flowers from March through May.