The ancient India was not much larger than the current state. In Śrīmad Bhāgavat 9.23.5-6 six provinces (viṣayān) are mentioned in the east (prācyakam) of Bhārata varṣa – Aṅga (the kingdom of Karṇa, now Bihar), Baṅga (west- and east-Bengal, of which the east is now a separate enclave inside India) and Kaliṅga (now Orissa). The six sons of Dīrghatamā became kings of the six provinces named after them.
baleḥ kṣetre mahīkṣitaḥ
cakruḥ sva-nāmnā viṣayān
ṣaḍ imān prācyakāṁś ca te
Jīva Goswāmī and Viśvanātha Cakravartī comment:
viṣayān deśān | prācyakān bhārata varṣasya pūrva-dig-vartinaḥ
« viṣayān means country or region, and prācyakān means on the eastern side of Bhārata Varṣa. »
So it is not that ancient India spread out to Vietnam or Indonesia and Vedic culture is now somehow lost there.
Also it is interesting that both Pāṇḍava-brides, Uttarā (princess of Matsya) and Draupadī (princess of Pancāla) came from close to Vraja.