I have seen many people calling Lord Krishna as 'Vasudev'. We know that Vasudev was the father of the Lord, but why is Krishna addressed using the same name?
First of all, Vāsudeva (with a long a) is the patronymic name of Krishna, what we would call a "last name". It means "son of Vasudeva" (with a short a). Second of all, long before the birth of Krishna, Vishnu has been known as Vāsudeva. How is this possible? It's because the name Vāsudeva has another meaning, given in this chapter of the Vishnu Purana:
The term Vásudeva means that all beings abide in that supreme being, and that he abides in all beings.
That's why Vishnu's four Vyuha forms were known as Vásudeva, Sankarshana, Pradyumna, and Aniruddha long before the birth of Krishna. Still, it's not just a coincidence that these are also the names of Krishna, Krishna's brother Balarama, Krishna's son Pradyumna, and Krishna's grandson Aniruddha, as I discuss here.