Syntax to add a variable for Row Number in R1C1 format

Hello, I have taken over a project half way done, and I was hoping someone coule help me out with the proper syntax for using a variable as the row number. In the code I am checking if there is an error, using vlookup to find values to search. I would like to use LastRow as the row number, rather than define a set range in the Vlookup:

c.offset(0,13).FormulaR1C1 = "=IF(ISERROR(VLOOKUP(RC[-10]," & WSName & "!R2C6:R[LastRow]C13,5,FALSE)),"""",VLOOKUP(RC[-10]," & WSName & "!R2C6:R[LastRow]C13,5,FALSE))"

I am having an issue with the quotes and the use of &'s, as the range I am searching is on another worksheet (WSName)... Any help much appreciated!
Thanks!