The Vedas clearly declare that the ultimate reality is one but we know him by different names. So, yes, oneness of Gods has been discussed in the Vedas themselves.
Suparnam viprAh kavyo vachobhir ekam santam bahudhA kalpayanti.
The wise seers describe the one existence (ekam santam) in various
words (bahudha vachobhih).
Rig Veda 1.114.5.
Ekam sad viprA bahudhA vadanti agnim yamam mAtrishvAnam Ahuh.
The sages describe the one existence (ekam sat) in many ways. It is
called as Agni, as Yama as Matarishvan.
Rig Veda 1.184.46.
NOTE- Regarding the Sakara-Nirakara thing i will update this answer later.