Rig-Veda or any other shashtra does not mention anywhere about the practice of the burning or burial of widows with their dead husbands.
युवं ह कृशं युवमश्विना शयुं युवं विधन्तं विधवामुरुष्यथः |
युवं सनिभ्य: स्तनयन्तमश्विनाप व्रजमूर्णुथः सप्तास्यम् || — ऋग्वेद (10-40-8)
A set of 14 Richa in 18th Mandala of the 10th Sukta deal with treatment of widows.
Read this thread to learn more about the status of:
Women in Manusmriti :
Women in Vedas :