CREATE TABLE virtuemart_product_customfields (
  virtuemart_customfield_id int(11) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT 'field id',
  virtuemart_product_id int(11) NOT NULL DEFAULT 0,
  virtuemart_custom_id int(11) NOT NULL DEFAULT 1 COMMENT 'custom group id',
  custom_value tinyint(4) NOT NULL COMMENT 'field value',
  PRIMARY KEY (virtuemart_customfield_id),
  INDEX idx_custom_value (custom_value),
  INDEX idx_virtuemart_custom_id (virtuemart_custom_id),  
  INDEX IDX_virtuemart_product_customf (virtuemart_custom_id, custom_value)
)
ENGINE = MYISAM
CHARACTER SET utf8
COLLATE utf8_general_ci;

EXPLAIN
SELECT
  virtuemart_product_id
FROM
  virtuemart_product_customfields
WHERE
  (virtuemart_custom_id  = 2 AND  custom_value  = 2);

EXPLAIN
SELECT
  virtuemart_product_id
FROM
  virtuemart_product_customfields
WHERE
  (virtuemart_custom_id  = 2 AND  custom_value  = 2)
  OR
  (virtuemart_custom_id  = 4 AND  custom_value  = 3);