Well there are so many details you're not telling us. During the winter, what are the conditions of your potted tree. Light, temperature, humidity, how long is it indoors, where do you live? All these things can make a big difference.

For some people, located in climates with a short growing season, they may have to put their citrus in a grow tent with a grow light for 4 months out of the year. Now usually you don't have to go to that extreme of a measure, but it's not a take it inside and forget about it either.

For one thing, keep the soil and roots from becoming too cold. It can get colder at times inside your house than you think. The second thing, using your heater can cause the tree to dry out. Cold air holds less moisture, so when you take that cold air and heat it, it can make the air dry. Third thing is light, citrus trees need light. If not to grow, then enough to survive. There's probably a lower light level in you house than you think. It's because your eyes adjust to lower light levels, so you don't realize it. How many hours of sun is that plant getting by the window in the Winter?

I don't know your situation, but if you really wanted to go all out you could hang a little cheap grow light over your plant, put a large clear plastic garbage bag covering the tree, and put a heat pad underneath the pot. Also make sure the soil is the type that retains water well. That's more important than many people realize. Then you could put some mulch over that, helps prevent that moist soil from drying out.

Soil should remain constantly moist but not too wet.

Plant only needs a little bit of fertilizer, don't expect more fertilizer to be a magical cure.

I grow indoors. Usually yellowing leaves means the soil needs less water and the surrounding air should have more humidity, and sometimes it means that the soil temperature has been going down too low at night. But I don't know if that experience will apply to you.

Magnesium deficiencies are often seen in acidic soils, that could also be causing leaf yellowing. Take it easy on the vinegar.

The leaves of a new flush are always more yellowish than mature leaves, this is normal. In a month of two the leaves normally mature and become a darker green.

Great comments, thank you. I live in the Tidewater area of Virginia. My Meyer lemon spends the winter months in our sunroom which, unfortunately, faces east. I think it is warm enough however. It usually blooms in February or so, but I have been unable to get lemons to set.

I used to live in Houston and had a Meyer in a pot outside year-round, covering it in times of freezing temperatures. I had lots of lemons. They are pretty hardy when they find their "sweet spot." It is trickier up here in Virginia, but I keep trying.