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I have a 10 inch RES in a 200 gallon outdoor pond. The pond is made of vinyl liner. Water is about 55 degrees and RES is not eating or moving around as much as usual. Will she hibernate or should I heat the water? If I should heat the water, what type of heater would anyone recommend?

Hello and welcome ,

Is it buried in ground ? If so what is your freeze line and or what climate zone you live in ? What is the deepest spot ? If above ground 200 gallons will react fast to the temps. I tried it above ground here in the south, used a sealed wall insulation around the outside and two 300 watt Eheim heaters ( both on just in case one went out ) also made a closed in spar accessible from only under water and watched every day. Depending on where you live it could work. May need a commercial heater and or deicer?

Most reading's will tell you do not let a captive turtle hibernate/ brumate ! We can never duplicate what nature offers a turtle fully to be safe.
You may want a simple container for indoors to over winter.

To "me" 55* is already too cold for a captive turtle , may want to decide soon. It's your call.

Is this her first year in the pond?

Don't heat the pond. If you heat the pond, the air temperature will still be cold enough to put her into shock or give her a RI. Either bring her inside or let her hibernate and don't bother her at all.