The access was granted by Red Mercury, a company owned by the Marange-Zimunya Community Share Ownership Trust, which has received an undertaking from the government of Zimbabwe for a licence to mine on the concession.
Vast will carry out initial due diligence on the area for at least two months to determine the value of entering into a joint venture for exploration, mining and marketing.
The principal terms of the JV have already been agreed, including paying annual dividends of at least 50% of profits; committing Vast to giving support to the local community and participating jointly with Red Mercury in cultural heritage ancestral activities; and promoting local jewellery manufacturing with part of the production.
Vast said the 15sq.km Heritage concession was close to its claim in the diamond fields and is thought to host an extension of the same geological system.
Vast Zimbabwe director Mark Mabhudhu said the concession area hadn't been mined before and was anticipated to contain economically viable diamondiferous alluvials, as well as conglomerate ore resources.
Vast CEO Andrew Prelea said, subject to results of the due diligence, the company was confident it would be able to move swiftly in building a valuable diamond arm of its business.
Vast previously signed a memorandum of understanding with Botswana Diamonds to develop diamond resources in the country.
Botswana Diamonds was looking forward to working with Vast to "realise the full potential of the area and others as they begin to emerge with Zimbabwe opening for business", according to chairman John Teeling.