Excluding specific rows based on content

Issue 1:

EXCEPT ( <LeftTable>, <RightTable> )

it excludes from LeftTable the rows also existing in RightTable; which means all columns of a row, i.e. VPT, DATE, EMP NAME and AMOUNT in your case, are taken into consideration. At a glimpse, for row 'Travel Claims Log'[VPT #]=61854, 'Travel Claims Log'[AMOUNT]=477.45 doesn't exist in 'RStars Travel', thus this row is kept.

Issue 2,

RStars Travel =
SELECTCOLUMNS(
    FILTER(
        RStarsTravelReport,
        NOT (
            LEFT( RStarsTravelReport[RSTravel_VPTNbr] ) = "C"
                || LEFT( RStarsTravelReport[RSTravel_VPTNbr], 2 ) = "LT"
        )
    ),
    "VPT #", RStarsTravelReport[RSTravel_VPTNbr],
    "Date", RStarsTravelReport[Final_Approv_Dt],
    "Employee Name", RStarsTravelReport[Vendor_Name],
    "Amount", MAX( RStarsTravelReport[Sfms_Trans_Amt], 0 )
)

filter out undesired rows with specific symbols upon creating the new table.