Is it ok to have Vitamin D levels just above the lower limit of normal? 

It depends! If you are just above the lower limit of normal and it is the start of the winter then I would be looking to optimise your level by taking a supplement as it is likely to drop further without strong sunlight exposure. If it is at the start of the summer months or you are expecting to go on a sunshine holiday and you will be spending lots of time outside then you will probably make enough vitamin D to optimise your levels. If for whatever reason you spend most of your time indoors or your skin is sensitive to the sun then you would be wise to continue a supplement of vitamin D all year round.  Don’t forget that vitamin D is fat soluble and best taken with a meal containing some fat so it will be better absorbed.