Whenever any contract for the purchase of real estate hereafter entered into shows that the title is to be taken by the vendees as joint tenants and not as tenants in common, and one or more of such vendees die before the delivery of the deed to such vendees, then in such case no heir, legatee or legal representative of such deceased vendees shall have or take any right, title or interest under the contract or in such real estate, but the obligation of the vendor or vendors under the contract (upon the performance thereof) shall be to convey such real estate to such surviving vendee or vendees, and if all such vendees die before such delivery then to the heirs or legatees of the last survivor; but nothing herein contained shall operate to change the effect of any assignment by any of the above stated vendees.