vcf-isec: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/vcf-isec - from package: vcftools

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/vcf-isec - from package: vcftools

You may install the required package with command apt-get install {package}

Error in other languages:
vcf-isec: bevel nie gevind nie
vcf-isec: comandă negăsită
vcf-isec: comando não encontrado
vcf-isec: commande introuvable
vcf-isec: command not found
vcf-isec: käsku ei ole
vcf-isec: không tìm thấy lệnh
vcf-isec: komanda nerasta
vcf-isec: Kommando nicht gefunden.
vcf-isec: kommandot finns inte
vcf-isec: komut yok
vcf-isec: nie znaleziono polecenia
vcf-isec: níor aimsíodh an t-ordú
vcf-isec: no se encontró la orden
vcf-isec: no s'ha trobat l'ordre
vcf-isec: opdracht niet gevonden
vcf-isec: parancs nem található
vcf-isec: perintah tidak ditemukan
vcf-isec: príkaz nenájdený
vcf-isec: příkaz nenalezen
vcf-isec: команда не найдена
vcf-isec: командата не е открита
vcf-isec: 命令找不到
vcf-isec: コマンドが見つかりません