Curious, what if you did the following (using David Burrows code):

WITH vv (VCHVendor,VGLVoucher)

AS (SELECT DISTINCT VCHVendor,VGLVoucher

FROM VoucherGLDist vgl

JOIN Voucher v ON v.VCHVoucherNbr = vgl.VGLVoucher

--WHERE vgl.VGLJEBatch = 21174

)

SELECT vv.VCHVendor, SUM(vgl.VGLAmount) AS [VGLAmount],

STUFF((SELECT ','+ b.VCHInvoiceNbr

FROM VoucherGLDist a

JOIN Voucher b ON b.VCHVoucherNbr = a.VGLVoucher

AND b.VCHVendor = vv.VCHVendor

--WHERE a.VGLJEBatch = 21174

GROUP BY a.VGLJEBatch,b.VCHVendor,b.VCHInvoiceNbr

FOR XML PATH ('')),1,1,'') AS [Invoices]

FROM VoucherGLDist vgl

JOIN vv ON vv.VGLVoucher = vgl.VGLVoucher

GROUP BY vgl.VGLJEBatch, vv.VCHVendor;