WHY BRAHMINS WERE CALLED VIPRA DURING ANCIENT TIMES

In ancient times, among the various terms used to describe Brahmins, one was Vipra. I came across this in two of the popular Sanskrit Slokas:

1.Sri Sastha Pancharatna Stotram (composer unknown)

विप्रपूज्यं विश्ववन्द्यं विष्णुशम्भोः प्रियं सुतम् ।

क्षिप्रप्रसादनिरतं शास्तारं प्रणमाम्यहम् ॥

Vipra pujyam Vishwa vandhyam Vishnu Shambho priyam sutam

Kshipra prasada niratam Sastharam pranamamyaham

Interpretation:

I bow in reverence before Lord Sastha, who is worshipped by learned Brahmins, adored by the universe, who is the beloved son of Vishnu and Shiva, and who is ready to grant  favors spontaneously to his devotees.

The term Vipra in the above verse refers to scholarly, highly knowledgeable Brahmins.

2.Sri Surya Mandala Stotram (composer Ved Vyas)

यन्मंडलं देवगणैः सुपूजितं

विप्रैः स्तुतं भावनमुक्तिकोविदम् ।

Yen Mandalam Deva Ganai Supujitam

Viprai Stutam bhavana Mukti kovitam

Interpretation 

Savita, the Sun god and his abode (mandalam) are worshipped by the Devas and Ganas. The learned Brahmins too worship Him to seek complete liberation which is granted by him

Here too the term Vipra stands for scholarly, pious Brahmins.

Talking of Surya Mandala Stotram, it is strikingly similar yet very different from Aditya Hridayam.

Both AH and SMS are associated with avatars of Lord Maha Vishnu -  Lord Ram and Sree Krishna. 

Both AH and SMS underline the importance of worship of Surya Deva, the Sun god.

Both AH and SMS were created before a Maha Yuddham.

While AH was preached by Sage Agastya to Lord Rama, SMS  appears directly from the mouth of Lord Krishna. 

While AH is an integral part of Ramayana itself it is not clearly known if SMS is a part of Mahabharata. It is found in the Bhavishyotara Puranam.

While AH is a preaching (Upadesam) SMS is a conversation (Samvadam).

Origin of the word Vipra

The earliest reference to the term Vipra appears in Rig Veda and is associated with two Rishis Vasishta and Kanva. 

विप्रं॒ विप्रा॒सोऽव॑से दे॒वं मर्ता॑स ऊ॒तये॑ । 

अ॒ग्निं गी॒र्भिर्ह॑वामहे ॥

We get clarity from Section 1 of  Shiva Purana titled Vidyeswara Samhita Chapter 13 called Sadachara (good conduct) about who can be called a Vipra.

Sage Suta in reply to a question replies

A brahmin learned in Vedas and of good conduct is called a Vipra. A brahmin endowed with only one of these two is a mere Dvija or the twice born.

(Suta is not a name but refers to a narrator. The original name of the sage is Ugrashravas, son of Sage Romaharshana and disciple of Ved Vyas . Suta got his name for narrating the Mahabharata)