Need help in increasing maximum table size
Sounds like MISSION: IMPOSSIBLE. Just kidding. I just cringe when I see MyISAM.
I can tell you what the MySQL Documentation says. Please note what it says:

You are using a MyISAM table and the space required for the table exceeds what is permitted by the internal pointer size. MyISAM permits data and index files to grow up to 256TB by default, but this limit can be changed up to the maximum permissible size of 65,536TB (2567 – 1 bytes).

With that in mind, you would have to change myisam_data_pointer_size to 7
First, add this to my.cnf under the [mysqld] group header
myisam_data_pointer_size=7

Then, run this in mysql
SET GLOBAL myisam_data_pointer_size = 7;
SET myisam_data_pointer_size = 7;

This will allow your current session and new connections to have the extended capacity and permit you to safely run those ALTER TABLE statements.
Give it a Try !!!
UPDATE 2014-12-03 00:51 EST
I was just thinking. The MyISAM tables vb_profilevisitor  and vb_post may not like the ALTER TABLE because they were created when myisam_data_pointer_size was 6.
To change it manually, do the following
CREATE TABLE vb_post_new LIKE vb_post;
ALTER TABLE  vb_post_new MAX_ROWS=98281474976710655 AVG_ROW_LENGTH=1000000000;
CREATE TABLE vb_profilevisitor_new LIKE vb_profilevisitor;
ALTER TABLE  MAX_ROWS=98281474976710655 AVG_ROW_LENGTH=1000000000;

Check the stats on vb_profilevisitor_new and vb_post_new
SELECT table_name,max_data_length,avg_row_length
FROM information_schema.tables
WHERE table_name IN ('vb_post_new','vb_profilevisitor_new');

If the max_data_length and avg_row_length changed, then copy the table data and swap new tables in
INSERT INTO vb_profilevisitor_new SELECT * FROM vb_profilevisitor;
RENAME TABLE
    vb_profilevisitor TO vb_profilevisitor_zap,
    vb_profilevisitor_new TO vb_profilevisitor
;
DROP vb_profilevisitor_zap;
INSERT INTO vb_post_new SELECT * FROM vb_post;
RENAME TABLE
    vb_post TO vb_post_zap,
    vb_post_new TO vb_post
;
DROP vb_profilevisitor_zap;