You might try BEDOPS bedops --ec:

$ bedops --ec --element-of 1 <(vcf2bed < a.vcf) <(sort-bed b.bed) > answer.bed

The use of --ec with bedops can return useful debugging information about malformed BED data, at the cost of a slower runtime. The --ec operator can be removed once the input is debugged.

It seems the problem is not related to bedtools but rather with your input files.

cat -t a.vcf doesn't fix the problem, rather it tells you whether your columns are tab delimited. With cat -t a.vcf (or cat -t b.bed) you should see on screen columns separated by ^I. Perhaps your columns are separated by spaces instead? If so, you could fix it with tr ' ' '\t' < a.vcf > A.vcf

Also, as the error message suggests, have a look if you coordinates are actually integers. Try to do sort -k2,2n a.vcf to see if the top rows (excluding the header) have integers in column 2.