Ли is a particle which marks an indirect question. It can NEVER start a sentence: "ли" is placed AFTER a word to which it is related (usually a verb, but not only a verb), and it is pronounced as a single word with the word it follows. It does not have a separate stress, and no pause is possible. E.g., "знает ли", "хочет ли", "приедет ли" are all pronounced as single words. Я спросил его, пойдёт ли он со мной. - I asked him whether (if) he would go with me. Я не знаю, будет ли завтра дождь. - I do not know whether (if) it will rain tomorrow. Если is a conjunction word which expresses conditions. Если ты не будешь работать, (то) я тебе не заплачу. = Я тебе не заплачу, если ты не будешь работать. (I will not pay you if you do not work). Если будет дождь, (то) мы останемся дома. = Мы останемся дома, если будет дождь. (We will stay at home if it rains).

The sentence means: "Did prince Vladimir choose the religion rightly?" Ли is like "Did" here, used for forming questions. Other examples: "Правда ли, что..." - "Is it true, that..." "Понимаете ли вы..." - "Do you understand..." etc.