You can try converting to IQueryable
then pulling out the Expression
property:
Dim expression = (From _row In table.Rows
Group _row By vGroup = _row("VENDOR")
Into VendorGroup = Group
Select New With {
Key vGroup,
.PI = VendorGroup.Sum(Function(r) r("PI")),
.ST = VendorGroup.Sum(Function(r) r("ST")),
.IS = VendorGroup.Sum(Function(r) r("IS")),
.RR = VendorGroup.Sum(Function(r) r("RR"))
}).AsQueryable()
.Expression
but the expression you get back may vary depending on the query provider you're using.