VEP will add the preceding bases to indels if you use the --database flag (in place of --cache which I assume you are currently using).
If you update your VEP installation (re-run the INSTALL.pl script to grab the latest API code), then it should now also work with --cache. Performance will also be improved if you have a FASTA file set up for sequence retrieval (the INSTALL.pl script can fetch this for you, or see http://www.ensembl.org/info/docs/tools/vep/script/vep_cache.html#fasta)
thank you guys for your kind replies!
I also found the hgvs library googling the internet.
I'll give it a chance. Wish me luck!