Is vacuum not same as Ākāśa?
No , Akasha or Space is not same as vacuum .Even from modern discoveries, its clear that empty space isn’t so empty , space is filled with invisible Dark-Matter , other low mass particles ,and background radiation.
How do Hindu scriptures define Ākāśa?
In Hinduism scriptures like Purusha Sukta from Rig-Veda and various puranas , Akasha or Antarikssam or space is described as part of “Virat Purusha” , which is first avatar of Paramatma or supreme soul.
नाभ्या आसीदन्तरिक्षं शीर्ष्णो द्यौः समवर्तत ।
पद्भ्यां भूमिर्दिशः श्रोत्रात्तथा लोकाँ अकल्पयन् ॥१४॥
Naabhyaa Aasiid-Antarikssam Shiirssnno Dyauh Samavartata | Padbhyaam Bhuumir-Dishah Shrotraat-Tathaa Lokaa Akalpayan ||14||
14.1: His Navel became the Antariksha (the intermediate Space between Heaven and Earth), His Head sustained the Heaven,
14.2: From His Feet the Earth (was sustained), and from His Ears the Directions (were sustained); in this manner all the Worlds were
regulated by Him.
द्वे जानुनी सुतलं विश्वमुर्ते रुरुद्वयम् वितलं चातलं च |
तज्जघनं महीपते नभस्तलं नाभिसरो गृणन्ति ||27||
dve jānunī sutalaṁ viśva-mūrter ūru-dvayaṁ vitalaṁ cātalaṁ ca
mahītalaṁ taj-jaghanaṁ mahīpate nabhastalaṁ nābhi-saro gṛṇanti
Meaning- The knees of the universal form are the planetary system of the name Sutala, and the two thighs are the Vitala and Atala
planetary systems. The hips are Mahītala, and outer space is the
depression of His navel SB 2.1.27
So from above shlokas we come to know that The space or Akasha itself is part of Virata purusha , which is everywhere in space. So space is different than a vacuum even in Hinduism scriptures.
We also find definition of Akasha in Brahma Sutras (Shankara Bhashya)
आकाशोऽर्थान्तरत्वादिव्यपदेशात् ॥ ४१॥
ākāśo'rthāntaratvādivyapadeśāt || 41 ||
ākāśaḥ—Âkâsa; arthāntaratvādi-vyapadeśāt—because it is declared to be
something different etc.
- Âkâsa (is Brahman) because it is declared to be something different etc. (from names and forms and yet their revealer).
“That which is called Âkâsa is the revealer of all names and forms.
That within which these names and forms are, is Brahman, the immortal,
the Self” (Chh. 8.14.1).
Here ‘Âkâsa’ is Brahman. Why ? Because names and forms are snid to be
within this Âkâsa, which is therefore different from these. In this
phenomenal world everything is conditioned by name and form, and
Brahman alone is beyond them. Âkâsa is said to be the revealer of
names and forms; and as the Inner Ruler of the whole world of names
and forms it cannot be anything else but Brahman. Moreover, epithets
like ‘Infinite’, ‘Immortal’, ‘Self’ also show that ‘Âkâsa’ here refers