There were many Parvati before present Parvati, which are same as present Parvati, and all were married to Lord Shiva.
So, the explicit mention of name Parvati also doesn't make a text late. As I discuss in my answer here and here , Same Lord Shiva is married with many Sati and many Parvati.
For eg. Shiva Purana gives it's example in RudraSamhita chapter 14 verse 9 as:
केचद्वदन्ति तां ज्येष्ठां मध्यमां चापरे शिवाम् ।
सर्वानन्तरजां केचित्कल्पभेदात्त्रयं च सत् ।।
Some say that Shivă (Sati) was eldest of them all, some say she was middle one and some wise treat her as youngest of all. All three opinions are correct, because of difference of Kalpa.
Similar is the case with Parvati. Parvati who killed Sumbha Nisumbha is not the same Parvati who is in Present time. I discuss this in the same answer here.
So, Sri Parvati is not one of the youngest Godesses. However present Parvati is one of the youngest Godesses.
But Does the name "Parvati" explicitly exists in Vedas?
No it is not. May be only in some few Upanishads.
Does this mean Parvati is not in Vedas?
No, it doesn't mean that. Parvati is mentioned by the name Uma.
For eg. ValmikiRamayana Balakanda narrates about conjugal relation of Shiva and Parvati, Birth of Skandha, Curse given by Parvati to Gods, etc... But it doesn't mention the name "Parvati" even a single time. All story is by the name Uma.
तस्याम् गंग इयम् अभवत् ज्येष्ठा हिमवतः सुता |
उमा नाम द्वितीया अभूत् कन्या तस्य एव राघव || १-३५-१६
"From Mena, this Ganga is emerged as an elder daughter to Himavanta, and a second daughter, by her name she is Uma, is also begotten by Himavanta, oh, Raghava... [1-35-16]
उग्रेण तपसा युक्ताम् ददौ शैलवरः सुताम् |
रुद्राय अप्रतिरूपाय उमाम् लोक नमस्कृताम् || १-३५-२१
"And that Uma who is unified with her rigorous sacrament thus became a reverential to all the worlds, and that best one among mountains, Himavanta, gave such a daughter to Rudra, an unequalled one in His aspect... [1-35-21]
And Lord Shiva is also called by the name "Umapati" and not Parvatipati. For eg:
अथ संवत्सरे पूर्णे सर्व लोक नमस्कृतः |
उमापतिः पशुपती राजानम् इदम् अब्रवीत् || १-४३-२
"On completion of one year, He who is venerated by all worlds and the consort of Uma and the god of creatures God Shiva, revealed Himself to the king and] spoke this to the king. [1-43-2]
So, Uma is name of Parvati. Actually Sati is called also by the name Uma. Hence the name Umapati means Consort of Parvati/Sati at the same time.
As we have seen above there are many Sati and many Parvati. So, Vedas use the term Uma to represent both. Hence, the name Parvati doesn't exist explicitly in Vedas.
For eg.. Lets see example of Kena Upanishad which is a principle Upanishad where Uma gives BrahmaVidya to Gods.
स तस्मिन्नेवाकाशे स्त्रियमाजगाम बहुशोभमानाम् उमां हैमवतीं ताँहोवाच किमेतद्यक्षमिति ॥ Kena Upanishad 3.12॥
There appeared in the sky that pretty girl, the Goddess of Wisdom, snowy Himalaya's daughter Uma. Light went to her and asked who was that mysterious Person.
The words used are "Uma Haimavatim" which means "Uma, the daughter of Himalayas." which is an explicit refrence for Parvati. For eg. ValmikiRamayana uses the word "Haimavati" several times to denote daughter of Himalayas:
इयम् हैमवती ज्येष्ठा गंगा हिमवतः सुता |
ताम् वै धारयितुम् राजन् हरः तत्र नियुज्यताम् ||
This is the snow-broth Ganga, the elder daughter of Himavanta, [the Himalaya,] and oh, king Bhageeratha, to sustain her [force in the course of her alighting onto earth,] Lord Shiva is to be designated there [on earth to receive her...][1-42-23]
Another refrence we come in YajurVeda Maitrayaniya Samhita 9.4 which states:
गिरिसुताय धीमहि तन्नो गौरी प्रचोदयात् ।
May daughter of mountains (Girisuta), Gauri enlighten our intellect.
Thus as it calls Girisuta and Gauri, it is also explicit refrence to Parvati.
In the Taittariya Aranyaka of YajurVeda, there is a hymn called Durga Sukta with Durga Gayatri which states:
कात्यायनाय विद्महे कन्याकुमारि धीमहि । तन्नो दुर्गिः
प्रचोदयात् ॥ [Yajurveda T.A. 10.1.7]
May we know Katyayana. For that, may we meditate upon Kanyakumari. May Durga impel us towards it.
Katyayani, Kanyakumari and Durga all are different forms and incarnations of Parvati.
In the Rudra Sukta of Taittariya Aranyaka of YajurVeda we find:
अम्बिकापतये उमापतये पशुपतये नमो नमः ॥ १॥ [YajurVeda T.A. 10.22.1]
Salutations and Salutations to Ambikapati (Lord of Ambika), Umapati (Lord of Uma) and Pasupati.
Here Uma can mean both Sati and Parvati and also eternal consort of Lord Shiva.
Sri Rudram of Yajurveda 8th Anuvaka starts with the verse:
नमस्सोमाय च रुद्राय च । [YajurVeda T.S. 4.5.8]
Salutations to him who is with Uma and who removes cry of Samsara (Rudra)
Here Somaaya means who is with Uma. Sa + Uma = Soma. This is explained by Sayana, AbhinavaGupta and BhattaBhaskara in their Sri Rudram Bhasya. It means who is with Uma because it is used in the format of specific name in the Anuvaka.
One of the most Ancient Upanishad Kaivalya Upanishad states:
उमासहायं परमेश्वरं प्रभुं त्रिलोचनं नीलकण्ठं प्रशान्तम् ।
ध्यात्वा मुनिर्गच्छति भूतयोनिं समस्तसाक्षिं तमसः परस्तात् ॥ 1.7॥
Meditating on the highest Lord, allied to Uma, powerful, three-eyed, blue-necked, and tranquil, the holy man reaches Him who is the source of all, the witness of all and is
beyond darkness (i.e. Avidya).
So, although the name "Parvati" doesn't appear explicitly in Vedas. But She is in Veda.
Regarding your other question whether the characters of Ramayana Mahabharata are mentioned in Vedas or not. Most of the Rishis of Ramayana and Mahabharata were seers of Veda Mantras. So they find mention as Rishi. For eg. Rishi Vasistha is the seer of Mahamrityunjaya Mantra of RigVeda, who is Guru of Rama in Ramayana.
There is no question of Updating of Vedas. If Vedas mention a place or name of anything then they are understood as eternal. In the next Manvantara or Kalpa a person with the same name will come coincidentally and do what Vedas state.