Solved it with eval and parse:
for(i in 1:length(vec)){
assign(paste("holder",i,sep=""),vec[i])
print ( paste ("holder",i,sep="") )
positions<-c(positions,grep( eval( parse(text= paste ("holder",i,sep="")) ),colnames(data),ignore.case=TRUE))
}
solved Pass sequentially named variables to functions within a loop in R