Misys Plc's agreement to be acquired at Europe's cheapest valuation for a software takeover is leaving the door open for rival bidders.

Vista Equity Partners said this week it will purchase Misys for 1.3 billion pounds ($2.1 billion), valuing the British financial software maker at 13 times earnings before interest, taxes, depreciation and amortization. That's the least expensive on record for a software takeover in Europe greater than $1 billion, according to data compiled by Bloomberg. At 21 percent more than Misys' 20-day trading average before merger talks with Temenos Group AG were disclosed last month, Vista's bid is also half the industry's average premium, the data show.

As banking regulations become stricter, Misys is seeking to sell more products that handle risk management, derivatives trading and lending. While Misys said the private equity firm's proposal is likely the "best deal," the stock is trading 1.7 percent above the cash bid of 350 pence a share, indicating some traders project a richer offer. Temenos, which ended talks over an all-share merger last week, can still make a counteroffer and Misys' biggest investor, ValueAct Capital, which teamed up with CVC Capital Partners, said it's weighing a possible joint bid.