My wife is pregnant now by 1 month and our first child is born on 21.01.2013 by C section so is it safe to go ahead

Also our 1 year daughter need at least 1 time breast feeding (at night while sleeping and in morning after get up) so is it safe to both of them because my wife taking pill (vitamine) as doctor advise

Please suggest

If it is just a vitamin, she should have those all along anyway, often. Depending on diet. Some women get pregnant, 2 or 3 months after birth, this is very normal.

Have you discussed any problems with her doctor?

If the doctor advises this than why are you questioning it? He knows far better than random internet people. Trust in your doctor unless you have a reason not to.

As was said, if it is just a normal vitamin or prenatal vitamin, that should be fine in regards to breastfeeding. What is the name of the vitamin?