I have a file with >19 000 rows with the following structure:

$ head -10 a_vt
9999.77,-83.03,-7.71771771772,276.97,-7.71771771772
9999.48,-83.57,-7.23723723724,276.43,-7.23723723724
9999.08,-83.99,-7.2972972973,276.01,-7.2972972973
9998.75,-81.71,-6.996996997,278.29,-6.996996997
9998.75,-81.65,-6.996996997,278.35,-6.996996997
9998.69,-80.87,-8.7987987988,279.13,-8.7987987988
9998.34,-81.05,-8.43843843844,278.95,-8.43843843844
9997.89,-83.99,-6.21621621622,276.01,-6.21621621622
9997.77,-77.27,-16.1261261261,282.73,-16.1261261261
9997.54,-82.43,-4.29429429429,277.57,-4.29429429429
...
...

and using this type of files (normally with a variable number of rows):

$ cat b_vm
22850,39.78686TN,39.78686TN,-75.6259,-14.9867,284.374,-14.9867
22901.9,9.90099TN,9.90099TN,-75.649,-14.9636,284.351,-14.9636
27742.2,160.0TN,160.0TN,-75.5999,-14.9922,284.4,-14.9922
22901.9,110.0TN,110.0TN,-75.6648,-14.9526,284.335,-14.9526
9998.69,110.0TN,110.0TN,-75.6551,-14.9496,284.345,-14.9496
9998.34,100.0TN,100.0TN,-75.62949999999998,-14.9573,284.37,-14.9573
27742.2,90.0TN,90.0TN,-75.60129999999998,-14.9973,284.399,-14.9973
27685.3,90.0TN,90.0TN,-75.6024,-14.9626,284.398,-14.9626
27742.2,80.0TN,80.0TN,-75.6014,-15.0006,284.399,-15.0006
22901.9,80.0TN,80.0TN,-75.6597,-14.9626,284.34,-14.9626

the 19k-rows file is filtered out after matching values of the first column in order to get:

$ awk 'NR==FNR { a[$1]; next }( ($1 in a) ) {print}' FS="," b_vm a_vt 
9998.69,-80.87,-8.7987987988,279.13,-8.7987987988
9998.34,-81.05,-8.43843843844,278.95,-8.43843843844

Not an awk expert here, but I understand that this one-liner awk call allows to get values from cache after comparing values of the first column in both files. The issue is to print the second column of b_vmt as well as follows:

9998.69,-80.87,-8.7987987988,279.13,-8.7987987988,**110.0TN**
9998.34,-81.05,-8.43843843844,278.95,-8.43843843844,**100.0TN**

Any hints are very welcomed,