How to keep a baby warm in cold weather?

I have to go out and buy some last minute groceries around noon today and it's not suppose to get above mid 20's with the car ride about 30 min away I was wondering if for a 5 month old a long sleeve cotton vest with a pair of socks and a thick cotton sleeper would be warm enough for the car ride and then put a micro plush throw around/over her on the walk from the car to the grocery store, I also have a micro plush swaddler that allows the legs to be separate for her car seat to be buckled if needed.