:~/Desktop/vanitygen-master$ make
cc -ggdb -O3 -Wall   -c -o vanitygen.o vanitygen.c
In file included from vanitygen.c:32:0:
pattern.h:54:12: error: field ‘vxc_bntarg’ has incomplete type
  BIGNUM    vxc_bntarg;
            ^~~~~~~~~~
pattern.h:55:12: error: field ‘vxc_bnbase’ has incomplete type
  BIGNUM    vxc_bnbase;
            ^~~~~~~~~~
pattern.h:56:12: error: field ‘vxc_bntmp’ has incomplete type
  BIGNUM    vxc_bntmp;
            ^~~~~~~~~
pattern.h:57:12: error: field ‘vxc_bntmp2’ has incomplete type
  BIGNUM    vxc_bntmp2;
            ^~~~~~~~~~
vanitygen.c: In function ‘vg_thread_loop’:
vanitygen.c:134:21: error: ‘BN_MASK2’ undeclared (first use in this function)
    if ((rekey_at == BN_MASK2) || (rekey_at > rekey_max))
                     ^~~~~~~~
vanitygen.c:134:21: note: each undeclared identifier is reported only once for each function it appears in
<builtin>: recipe for target 'vanitygen.o' failed
make: *** [vanitygen.o] Error 1

Take your decimal number.
I like this one: 1,125,899,906,842,623 because it converts to this

11111111111111111111111111111111111111111111111111 binary number

but you need to convert it to this

3FFFFFFFFFFFF hexadecimal number

then pad it with zeros to make it 64 characters

0000000000000000000000000000000000000000000000000003FFFFFFFFFFFF

now add 80 to the beginning (to make it a Bitcoin private key)

hash it once (SHA256)

08260BC6AE17D00E40464A8EE0DA4C9FDAF1BEFC14C528CFC169798D868860DD

hash that result  (SHA256)

53FA3A525E44DEF51F9A0B6E910E6DCA51AB0A99E1756BB97BE32E01308390A0

Now take the first 4 bytes (this is the checksum) and add it to the end of the red number

800000000000000000000000000000000000000000000000000003FFFFFFFFFFFF53FA3A52

convert that to Base58

5HpHagT65TZzG1PH3CSu63k8DbpvD8s5ip4nFAaVmEdoLi3nb9s