Question: In Vedas, we find the name of the Vedic God, Vishnu. However, in Puranas, the genealogies of Kings and Sages trace back to Mahavishnu. From Mahavishnu, Brahma was created. Then, Brahma created his Manasaputras. Most probably, they were 10 in numbers. From them, mankind was created. Now, are Vishnu and Mahavishnu indicate the same thing? If they are the same, what is the point of having two different names?

Answer: 
Vishnu is the Preserver of the Universe.
He is the part of the Trimurthi along with Bramha and Shiva.

But Mahavishnu is the origin of the Trimurthi including Vishnu according to Bramha Samhita.

> 
> The same **Mahā-Viṣṇu created Viṣṇu from His left limb**, Brahmā, the
> first progenitor of beings, from His right limb and, from the space
> between His two eyebrows, Śambhu, the divine masculine manifested
> halo.
> -Śrī brahma-saṁhitā 5.15
> 
> 
> 

But actually Vishnu and Maha Vishnu are not different from each other.
Mahavishnu [also called Kāraṇodakaśāyī-Viṣṇu.] is an expansion of Vishnu. Mahavishnu rests and from his body emerge all the universes as golden bubbles. Lord Mahavishnu is the amsa of Lord Krishna. But Krishna is the supreme personality of godhead.

Mahavishnu divides Himself into parts then turns into smaller Virāṭs in every universe. Brahmās spring from every small Virāṭs and create the cosmos. From the forehead Brahmās spring eleven Rudras for the destruction of the creation. In Sātvata-tantra, the small Virāṭ is called Garbhodakaśāyī-Viṣṇu
In Brahma vaivarta purana, Krishna janmakhanda, chapter 94.59:

> 
> mahad vishnor lomakupe tadevam ca anu avasthitam
> tasya yavanti lomani tani vishvani samti ca
> tasya yavanti lomani tani vishvani samti ca
> 
> 
> 

“The Lord Mahavishnu sustains as many universes in him,as there are number of pores of hair on his body. They all are situated like the small atomic particles in him. Even that lord Mahavishnu is the one sixteenth portion(amsa) of Lord Krishna.”

Brahma samhita 5.48 also echoes with the above statement as follows:

> 
> yasyaika-niśvasita-kālam athāvalambya
> jīvanti loma-vilajā jagad-aṇḍa-nāthāḥ
> viṣṇur mahān sa iha yasya kalā-viśeṣo
> govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
> 
> 
> 

“Brahma and other lords of the worlds appear from the pores of hair of Mahavishnu, remain alive as long as the duration of one exhalation of the him. I adore the primeval Lord Govinda of whose subjective personality MahaViṣhnu is the portion of portion.”